[clang] [Clang] Avoid an extra `FunctionPrototypeScope` for lambda trailing requires-clauses (PR #194068)

[Erich Keane via cfe-commits]

https://github.com/erichkeane approved this pull request.

I have actually seen this problem a few times, so glad you figured out the cause!  This looks right to me, but give others a day or two a chance to look too.

https://github.com/llvm/llvm-project/pull/194068