[clangd] Fix link failures for Preprocessor::addCommentHandler

Summary:
D42640 adds calls to `Preprocessor::addCommentHandler` in
`unittests/clangd/SymbolCollectorTests.cpp` and
`clangd/global-symbol-builder/GlobalSymbolBuilderMain.cpp` but does not
link `clangLex` library. This causes undefined reference errors when
built with `-DBUILD_SHARED_LIBS=ON`.

Reviewers: ioeric

Subscribers: klimek, mgorny, ilya-biryukov, jkorous-apple, cfe-commits

Differential Revision: https://reviews.llvm.org/D43437

llvm-svn: 325458