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

[Fangrui Song via llvm-commits]

MaskRay wrote:

Context:

https://reviews.llvm.org/D142318 (2023) introduced `PerThreadBumpPtrAllocator`. It's only used by DWARFLinker. I don't recommend using this elsewhere - lld uses its own `getSpecificAllocSingletonThreadLocal`.

**caller participation** is a mechanism in work-stealing schedulers where a thread initiating a parallel block joins in executing the spawned tasks instead of sitting idle.

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