[clangd] Set thread priority on Windows

clangd/Threading.cpp

 #include "Threading.h"
 #include "Trace.h"
 #include "llvm/ADT/ScopeExit.h"
 #include "llvm/Support/FormatVariadic.h"
 #include "llvm/Support/Threading.h"
 #include <atomic>
 #include <thread>
 #ifdef __USE_POSIX
 #include <pthread.h>
 #elif defined(__APPLE__)
 #include <sys/resource.h>
+#elif defined (_WIN32)
+#include <Windows.h>
 #endif
 
 namespace clang {
 namespace clangd {
 
 void Notification::notify() {
   {
     std::lock_guard<std::mutex> Lock(Mu);
pthread_setschedparam(
                                                                 : SCHED_OTHER,
       &priority);
 #elif defined(__APPLE__)
   // https://developer.apple.com/library/archive/documentation/System/Conceptual/ManPages_iPhoneOS/man2/getpriority.2.html
   setpriority(PRIO_DARWIN_THREAD, 0,
               Priority == ThreadPriority::Low && !AvoidThreadStarvation
                   ? PRIO_DARWIN_BG
                   : 0);
+#elif defined(_WIN32)
+  SetThreadPriority(GetCurrentThread(),
+                    Priority == ThreadPriority::Low && !AvoidThreadStarvation
+                        ? THREAD_MODE_BACKGROUND_BEGIN
+                        : THREAD_MODE_BACKGROUND_END);
 #endif
 }
 
 void preventThreadStarvationInTests() { AvoidThreadStarvation = true; }
 
 } // namespace clangd
 } // namespace clang