Add hardware_concurrency interface to llvm::thread (NFC)

Part of D15390.

llvm-svn: 262677

llvm/include/llvm/Support/thread.h

@@ -57,6 +57,7 @@ struct thread {
   thread(const thread &) = delete;
 
   void join() {}
+  static unsigned hardware_concurrency() { return 1; };
 };
 
 }