[Syntax] Add mapping from spelled to expanded tokens for TokenBuffer

Summary:
Same restrictions apply as in the other direction: macro arguments are
not supported yet, only full macro expansions can be mapped.

Taking over from https://reviews.llvm.org/D72581.

Reviewers: gribozavr2, sammccall

Reviewed By: gribozavr2

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77209