[clangd] Fix tests build for GCC5

Summary:
Build log:
```
llvm-project/clang-tools-extra/clangd/unittests/PreambleTests.cpp: In member function ‘virtual void clang::clangd::{anonymous}::PreamblePatchTest_Define_Test::TestBody()’:
llvm-project/clang-tools-extra/clangd/unittests/PreambleTests.cpp:267:3: error: could not convert ‘(const char*)"\012        #define BAR\012        [[BAR]]"’ from ‘const char*’ to ‘llvm::StringLitera ’
   };
   ^
llvm-project/clang-tools-extra/clangd/unittests/PreambleTests.cpp:267:3: error: could not convert ‘(const char*)"#line 0 \".*main.cpp\"\012#line 2\012#define         BAR\012"’ from ‘const char*’ to ‘llvm::StringLiteral’
llvm-project/clang-tools-extra/clangd/unittests/PreambleTests.cpp:267:3: error: could not convert ‘(const char*)"\012        #define BAR \\\012\012        [[BAR]]"’ from ‘const char*’ to ‘llvm::StringLiteral’
llvm-project/clang-tools-extra/clangd/unittests/PreambleTests.cpp:267:3: error: could not convert ‘(const char*)"#line 0 \".*main.cpp\"\012#line 2\012#define         BAR\012"’ from ‘const char*’ to ‘llvm::StringLiteral’
llvm-project/clang-tools-extra/clangd/unittests/PreambleTests.cpp:267:3: error: could not convert ‘(const char*)"\012        #define \\\012                BAR\012        [[BAR]]"’ from ‘const char*’ to ‘llvm::StringLiteral’
llvm-project/clang-tools-extra/clangd/unittests/PreambleTests.cpp:267:3: error: could not convert ‘(const char*)"#line 0 \".*main.cpp\"\012#line 3\012#define         BAR\012"’ from ‘const char*’ to ‘llvm::StringLiteral’
llvm-project/clang-tools-extra/clangd/unittests/PreambleTests.cpp: In member function ‘virtual void clang::clangd::{anonymous}::PreamblePatchTest_LocateMacroAtWorks_Test::TestBody()’:
llvm-project/clang-tools-extra/clangd/unittests/PreambleTests.cpp:357:3: error: could not convert ‘(const char*)""’ from ‘const char*’ to ‘llvm::StringLiteral’
   };
   ^
llvm-project/clang-tools-extra/clangd/unittests/PreambleTests.cpp:357:3: error: could not convert ‘(const char*)"\012            #define $def^FOO\012            $use^FOO"’ from ‘const char*’ to ‘llvm::StringLiteral’
llvm-project/clang-tools-extra/clangd/unittests/PreambleTests.cpp:357:3: error: could not convert ‘(const char*)""’ from ‘const char*’ to ‘llvm::StringLiteral’
llvm-project/clang-tools-extra/clangd/unittests/PreambleTests.cpp:357:3: error: could not convert ‘(const char*)"\012            #define $def^FOO\012            #undef $use^FOO"’ from ‘const char*’ to ‘llvm::StringLiteral’
llvm-project/clang-tools-extra/clangd/unittests/PreambleTests.cpp:357:3: error: could not convert ‘(const char*)""’ from ‘const char*’ to ‘llvm::StringLiteral’
llvm-project/clang-tools-extra/clangd/unittests/PreambleTests.cpp:357:3: error: could not convert ‘(const char*)"\012            #define $def^FOO\012            #undef FOO\012            $use^FOO"’ from ‘const char*’ to ‘llvm::StringLiteral’
llvm-project/clang-tools-extra/clangd/unittests/PreambleTests.cpp:357:3: error: could not convert ‘(const char*)""’ from ‘const char*’ to ‘llvm::StringLiteral’
llvm-project/clang-tools-extra/clangd/unittests/PreambleTests.cpp:357:3: error: could not convert ‘(const char*)"\012            #define \\\012              $def^FOO\012            $use^FOO"’ from ‘const char*’ to ‘llvm::StringLiteral’
llvm-project/clang-tools-extra/clangd/unittests/PreambleTests.cpp:357:3: error: could not convert ‘(const char*)""’ from ‘const char*’ to ‘llvm::StringLiteral’
llvm-project/clang-tools-extra/clangd/unittests/PreambleTests.cpp:357:3: error: could not convert ‘(const char*)"\012            #\\\012              define /* FOO */\\\012              /* FOO */ $def^FOO\012            $use^FOO"’ from ‘const char*’ to ‘llvm::StringLiteral’
llvm-project/clang-tools-extra/clangd/unittests/PreambleTests.cpp:357:3: error: could not convert ‘(const char*)"#define FOO"’ from ‘const char*’ to ‘llvm::StringLiteral’
llvm-project/clang-tools-extra/clangd/unittests/PreambleTests.cpp:357:3: error: could not convert ‘(const char*)"\012            #define BAR\012            #define $def^FOO\012            $use^FOO"’ from ‘const char*’ to ‘llvm::StringLiteral’
llvm-project/clang-tools-extra/clangd/unittests/PreambleTests.cpp: In member function ‘virtual void clang::clangd::{anonymous}::PreamblePatchTest_RefsToMacros_Test::TestBody()’:
llvm-project/clang-tools-extra/clangd/unittests/PreambleTests.cpp:445:3: error: could not convert ‘(const char*)""’ from ‘const char*’ to ‘llvm::StringLiteral’
   };
   ^
llvm-project/clang-tools-extra/clangd/unittests/PreambleTests.cpp:445:3: error: could not convert ‘(const char*)"\012            #define ^FOO\012            ^[[FOO]]"’ from ‘const char*’ to ‘llvm::StringLiteral’
llvm-project/clang-tools-extra/clangd/unittests/PreambleTests.cpp:445:3: error: could not convert ‘(const char*)"#define FOO"’ from ‘const char*’ to ‘llvm::StringLiteral’
llvm-project/clang-tools-extra/clangd/unittests/PreambleTests.cpp:445:3: error: could not convert ‘(const char*)"\012            #define BAR\012            #define ^FOO\012            ^[[FOO]]"’ from ‘const char*’ to ‘llvm::StringLiteral’
llvm-project/clang-tools-extra/clangd/unittests/PreambleTests.cpp:445:3: error: could not convert ‘(const char*)""’ from ‘const char*’ to ‘llvm::StringLiteral’
llvm-project/clang-tools-extra/clangd/unittests/PreambleTests.cpp:445:3: error: could not convert ‘(const char*)"\012            #define ^FOO\012            #undef ^FOO"’ from ‘const char*’ to ‘llvm::StringLiteral’
llvm-project/clang-tools-extra/clangd/unittests/PreambleTests.cpp: In member function ‘virtual void clang::clangd::{anonymous}::PreamblePatch_ModifiedBounds_Test::TestBody()’:
llvm-project/clang-tools-extra/clangd/unittests/PreambleTests.cpp:512:3: error: could not convert ‘(const char*)""’ from ‘const char*’ to ‘llvm::StringLiteral’
   };
   ^
llvm-project/clang-tools-extra/clangd/unittests/PreambleTests.cpp:512:3: error: could not convert ‘(const char*)"\012            #define FOO\012            FOO"’ from ‘const char*’ to ‘llvm::StringLiteral’
llvm-project/clang-tools-extra/clangd/unittests/PreambleTests.cpp:512:3: error: could not convert ‘(const char*)"#define FOO"’ from ‘const char*’ to ‘llvm::StringLiteral’
llvm-project/clang-tools-extra/clangd/unittests/PreambleTests.cpp:512:3: error: could not convert ‘(const char*)"#define BAR"’ from ‘const char*’ to ‘llvm::StringLiteral’
llvm-project/clang-tools-extra/clangd/unittests/PreambleTests.cpp:512:3: error: could not convert ‘(const char*)"\012            #define FOO\012            #undef FOO"’ from ‘const char*’ to ‘llvm::StringLiteral’
llvm-project/clang-tools-extra/clangd/unittests/PreambleTests.cpp:512:3: error: could not convert ‘(const char*)"#define FOO"’ from ‘const char*’ to ‘llvm::StringLiteral’
```

Patch by @ArcsinX !

Reviewers: kadircet, sammccall

Reviewed By: kadircet

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D83548