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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

This change appears to have introduced a regression: https://github.com/llvm/llvm-project/issues/114815 with fairly trivial code:
```
int decltype;
```
Any chance you can investigate @sdkrystian?

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