[llvm] PerThreadBumpPtrAllocator: remove dependency on getThreadIndex (PR #209687)

[Fangrui Song via llvm-commits]

MaskRay wrote:

@aganea Thank you for approving the follow-up https://github.com/llvm/llvm-project/pull/209690 (what I actually need). Would you approve this as well? :)

https://github.com/llvm/llvm-project/pull/209687