[clang] [Clang][Parse] Fix ambiguity with nested-name-specifiers that may declarative (PR #96364)

[Krystian Stasiowski via cfe-commits]

sdkrystian wrote:

I think this is ready to merge. I have [a branch](https://github.com/sdkrystian/llvm-project/tree/perf/backtrack-raii) which fully implements annotated/unannotated preprocessor backtrack using RAII objects, but I think this is beyond the scope of this PR.

https://github.com/llvm/llvm-project/pull/96364