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

[Dan Liew via cfe-commits]

delcypher wrote:

@pdherbemont I've landed on your behalf. I tweaked the commit message to be more descriptive of the change we finally landed and noted that you are the author of this patch. Thanks again for working on this :) 

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