[clang] [Clang] Fix double revert of a keyword token that was cached before the first fallback (PR #219799)

[Akash Manna via cfe-commits]

akash-manna-sky wrote:

> I was thinking we might already have tests for these traits. If it's convenient, you could just move the tests over—there's no need to create a new file. (Alternatively, I can open a PR for it myself someday; this won't be a blocker for merging.)

I kept as a separate file for now..

https://github.com/llvm/llvm-project/pull/219799