[coroutines] Add overloaded unary 'operator co_await'.

llvm-svn: 250991
diff --git a/clang/lib/Sema/SemaOverload.cpp b/clang/lib/Sema/SemaOverload.cpp
index 743ca68..d4a2200 100644
--- a/clang/lib/Sema/SemaOverload.cpp
+++ b/clang/lib/Sema/SemaOverload.cpp
@@ -8228,6 +8228,7 @@
   case OO_Array_New:
   case OO_Array_Delete:
   case OO_Call:
+  case OO_Coawait:
     llvm_unreachable(
                     "Special operators don't use AddBuiltinOperatorCandidates");