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

[Krystian Stasiowski via cfe-commits]

sdkrystian wrote:

@mizvekov My initial implementation used the existing `UnannotatedTentativeParsingAction` implementation, but that had a _significant_ performance hit (7% vs 0.01%) compared to the new implementation. My new implementation of `UnannotatedTentativeParsingAction` is also more versatile, as it allows unbounded, unannotated tentative parsing _without_ having to cache tokens up to a specific token that is known to be past the construct being tentatively parsed (in my initial implementation, that token was a `{` or `;`). 

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