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

[Nuri Amari via llvm-commits]

NuriAmari wrote:

> Have you measured the compile time if the max concurrency is 1 or a small value - wondering if there is overhead?

I haven't, but I'll give it a try. 

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