[clangd] Fix -DBUILD_SHARED_LIBS=ON build - SwapIfBranches needs clangAST.

Else, fails with:

[1/2] Linking CXX executable bin/clangd
FAILED: bin/clangd
: && /usr/bin/g++  -pipe -O2 -g0 -UNDEBUG -fPIC -fvisibility-inlines-hidden -Werror=date-time -std=c++11 -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wno-missing-field-initializers -pedantic -Wno-long-long -Wimplicit-fallthrough -Wno-maybe-uninitialized -Wno-class-memaccess -Wno-noexcept-type -Wdelete-non-virtual-dtor -Wno-comment -fdiagnostics-color -ffunction-sections -fdata-sections -fno-common -Woverloaded-virtual -fno-strict-aliasing -pipe -O2 -g0 -UNDEBUG  -fuse-ld=lld -Wl,--color-diagnostics -Wl,-allow-shlib-undefined     -Wl,-O3 -Wl,--gc-sections tools/clang/tools/extra/clangd/refactor/tweaks/CMakeFiles/obj.clangDaemonTweaks.dir/SwapIfBranches.cpp.o tools/clang/tools/extra/clangd/tool/CMakeFiles/clangd.dir/ClangdMain.cpp.o  -o bin/clangd  -Wl,-rpath,"\$ORIGIN/../lib" lib/libLLVMSupport.so.9svn -lpthread lib/libclangBasic.so.9svn lib/libclangTidy.so.9svn lib/libclangDaemon.so.9svn lib/libclangFormat.so.9svn lib/libclangFrontend.so.9svn lib/libclangSema.so.9svn lib/libclangTooling.so.9svn lib/libclangToolingCore.so.9svn && :
ld.lld: error: undefined symbol: clang::FunctionDecl::getBody(clang::FunctionDecl const*&) const
>>> referenced by SwapIfBranches.cpp
>>>               tools/clang/tools/extra/clangd/refactor/tweaks/CMakeFiles/obj.clangDaemonTweaks.dir/SwapIfBranches.cpp.o:(clang::FunctionDecl::getBody() const)

and so on.

llvm-svn: 352841