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

[Arthur Eubanks via cfe-commits]

aeubanks wrote:

reduced:
```
struct Lock {};
struct A {
        Lock lock;
        union {
                bool b __attribute__((guarded_by(lock)));
        };
};
```

seems like a regression, can we revert for now?

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