[llvm] [llvm/Support] Make `llvm::sys::RWMutex` Lockable (PR #90667)

[Med Ismail Bennani via llvm-commits]

medismailben wrote:

> as an aside - per the instructions here ( https://llvm.org/docs/GitHub.html#updating-pull-requests ) please don't force-push to pull request reviews, as it makes it difficult to trace back comments and discussion through the revisions of a patch (I think, especially, inline comments can get lost/hidden/hard to find when force pushing)

Sure! I'll keep that in mind next time.

https://github.com/llvm/llvm-project/pull/90667