[Syntax] Merge overlapping top-level macros in TokenBuffer

Summary:
Our previous definition of "top-level" was too informal, and didn't
allow for overlapping macros that each directly produce expanded tokens.
See D77507 for previous discussion.

Fixes http://bugs.llvm.org/show_bug.cgi?id=45428

Reviewers: kadircet, vabridgers

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77615