[llvm] [Support] Remove unused includes (NFC) (PR #116752)

[Florian Hahn via llvm-commits]

fhahn wrote:

Looks like this breaks building on macOS

```
llvm/lib/Support/RWMutex.cpp:47:36: error: use of undeclared identifier 'safe_malloc'
   47 |     static_cast<pthread_rwlock_t*>(safe_malloc(sizeof(pthread_rwlock_t)));
```

https://github.com/llvm/llvm-project/pull/116752