[clangd] Collecting main file macro expansion locations in ParsedAST.

Summary: TokenBuffer does not collect macro expansions inside macro arguments which is needed for semantic higlighting. Therefore collects macro expansions in the main file in a PPCallback when building the ParsedAST instead.

Reviewers: hokein, ilya-biryukov

Subscribers: MaskRay, jkorous, arphaman, kadircet, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66928

llvm-svn: 370452