[llvm] Make WriteIndexesThinBackend multi threaded (PR #109847)

[Teresa Johnson via llvm-commits]

https://github.com/teresajohnson commented:

Thanks for this patch. I have a few suggestions below. Have you measured the compile time if the max concurrency is 1 or a small value - wondering if there is overhead?

https://github.com/llvm/llvm-project/pull/109847