Implement __underlying_type for libc++.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@131633 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Parse/ParseExprCXX.cpp b/lib/Parse/ParseExprCXX.cpp
index 7410dbe..0862af3 100644
--- a/lib/Parse/ParseExprCXX.cpp
+++ b/lib/Parse/ParseExprCXX.cpp
@@ -962,6 +962,7 @@
   case tok::kw_bool:
   case tok::kw_decltype:
   case tok::kw_typeof:
+  case tok::kw___underlying_type:
     return true;
 
   default: