[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