message: Set indexer threads' priority to ThreadPriority::Background
diff: #include "working_files.hh"
  #include <llvm/ADT/Twine.h>
  +#include <llvm/Config/llvm-config.h>
  #include <llvm/Support/Threading.h>
  #include <rapidjson/document.h>
  @@ -255,6 +256,12 @@ void *indexer(void *arg_) {
  delete arg;
  std::string name = "indexer" + std::to_string(idx);
  set_thread_name(name.c_str());
  + // Don't lower priority on __APPLE__. getpriority(2) says "When setting a
  + // thread into background state the scheduling priority is set to lowest
  + // value, disk and network IO are throttled."
  +#if LLVM_ENABLE_THREADS && LLVM_VERSION_MAJOR >= 9 && !defined(__APPLE__)
  + set_thread_priority(ThreadPriority::Background);
  +#endif
  pipeline::indexer_Main(h->manager, h->vfs, h->project, h->wfiles);
  pipeline::threadLeave();
  return nullptr;
  