[llvm] Windows: use EcoQoS for ThreadPriority::Background (PR #148797)

[Martin Storsjö via llvm-commits]

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@@ -107,6 +107,31 @@ void llvm::get_thread_name(SmallVectorImpl<char> &Name) {
 }
 
 SetThreadPriorityResult llvm::set_thread_priority(ThreadPriority Priority) {
+
+  typedef BOOL(WINAPI * SetThreadInformation_t)(
+      HANDLE hThread, THREAD_INFORMATION_CLASS ThreadInformationClass,
+      _In_reads_bytes_(ThreadInformationSize) PVOID ThreadInformation,
+      ULONG ThreadInformationSize);
+  static const auto pfnSetThreadInformation =
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mstorsjo wrote:

I think the intent of using `GetProcAddress` here is so that we don't have a hard dependency on the symbol being present. LLVM currently works down to Windows 7, while this function only is available since Windows 8.

https://github.com/llvm/llvm-project/pull/148797