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

[Nuri Amari via llvm-commits]

NuriAmari wrote:

These measurements were taken with a `RelWithDebInfo` build of the toolchain, but I think it should serve to illustrate we shouldn't be too worried about overhead. 

Without this patch all together I measured:

```
real    10m38.836s
user    9m57.354s
sys     0m47.409s
```

With this patch but with the thread pool hard coded to use max concurrency = 1:

```
real    11m11.859s
user    10m29.944s
sys     0m49.081s
```

With this patch but with the thread pool hard coded to use max concurrency = 2:

```
real    6m24.810s
user    10m1.542s
sys     0m48.052s
```

I imagine the overhead would be reduced with a release build, and we still see benefit from 2 threads onwards.


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