[Syntax] Added function to get macro expansion tokens to TokenBuffer.

Summary:
Returns the first token in every mapping where the token is an identifier.
This API is required to be able to highlight macro expansions in clangd.

Reviewers: hokein, ilya-biryukov

Subscribers: kadircet, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66470

llvm-svn: 369385