[clang] Support [[guarded_by(mutex)]] attribute inside C struct (PR #94216)

[Pierre d'Herbemont via cfe-commits]

pdherbemont wrote:

So I found the cause of the regression: We used to parse the argument in an `Unevaluated` context in C++ mode. We now parse them in a `PotentiallyEvaluated` context. Switching to `Unevaluated` makes the C parsing code fail. A simple fix is to switch the context based on the language detected.

https://github.com/llvm/llvm-project/pull/94216