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

[Abhina Sree via llvm-commits]

abhina-sree wrote:

This patch is causing the following error on z/OS 
```
llvm/include/llvm/Support/PerThreadBumpPtrAllocator.h:75:12: error: thread-local storage is not supported for the current target
   75 |     static thread_local std::vector<AllocatorTy *> Cache;
      |            ^
```

Can we guard the usage of thread_local behind LLVM_ENABLE_THREADS or __has_feature(cxx_thread_local) ? Thanks in advance

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