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/Sema/SemaTemplateVariadic.cpp b/lib/Sema/SemaTemplateVariadic.cpp
index 096d353..100c6a7 100644
--- a/lib/Sema/SemaTemplateVariadic.cpp
+++ b/lib/Sema/SemaTemplateVariadic.cpp
@@ -617,7 +617,8 @@
   const DeclSpec &DS = D.getDeclSpec();
   switch (DS.getTypeSpecType()) {
   case TST_typename:
-  case TST_typeofType: {
+  case TST_typeofType:
+  case TST_underlying_type: {
     QualType T = DS.getRepAsType().get();
     if (!T.isNull() && T->containsUnexpandedParameterPack())
       return true;