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

[Alexandre Ganea via llvm-commits]

================
@@ -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 =
+      (SetThreadInformation_t)GetProcAddress(
+          GetModuleHandle(TEXT("kernel32.dll")), "SetThreadInformation");
+
+  if (pfnSetThreadInformation) {
+    if (Priority == ThreadPriority::Background) {
----------------
aganea wrote:

I think you should also cover the "else" case here, if the code decides to put back the thread in a higher priority state. In that case, we should revert the throttling by calling `SetThreadInformation` again with `.StateMask` set to zero.

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