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

[Nuri Amari via llvm-commits]

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@@ -87,6 +87,7 @@ BitcodeCompiler::BitcodeCompiler() {
   auto onIndexWrite = [&](StringRef S) { thinIndices.erase(S); };
   if (config->thinLTOIndexOnly) {
     backend = lto::createWriteIndexesThinBackend(
+        llvm::hardware_concurrency(config->thinLTOJobs),
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NuriAmari wrote:

I tried `heavyweight_hardware_concurrency` as well, but I didn't see a noticeable difference. 

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