Fix memory leak in DeclTest.

Fixes a leak introduced in r372903, detected on the ASan bot.
http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux-fast/builds/35430/steps/check-clang%20asan/logs/stdio

Direct leak of 192 byte(s) in 1 object(s) allocated from:
    #0 0x561d88 in operator new(unsigned long) /b/sanitizer-x86_64-linux-fast/build/llvm-project/compiler-rt/lib/asan/asan_new_delete.cc:105
    #1 0x1a48779 in clang::ItaniumMangleContext::create(clang::ASTContext&, clang::DiagnosticsEngine&) /b/sanitizer-x86_64-linux-fast/build/llvm-project/clang/lib/AST/ItaniumMangle.cpp:5134:10
    #2 0xdff000 in Decl_AsmLabelAttr_Test::TestBody() /b/sanitizer-x86_64-linux-fast/build/llvm-project/clang/unittests/AST/DeclTest.cpp:97:23

llvm-svn: 372925