Implement __underlying_type for libc++.

llvm-svn: 131633
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 7c6cde2..fd28f2b 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -2864,7 +2864,8 @@
   switch (DS.getTypeSpecType()) {
   case DeclSpec::TST_typename:
   case DeclSpec::TST_typeofType:
-  case DeclSpec::TST_decltype: {
+  case DeclSpec::TST_decltype:
+  case DeclSpec::TST_underlying_type: {
     // Grab the type from the parser.
     TypeSourceInfo *TSI = 0;
     QualType T = S.GetTypeFromParser(DS.getRepAsType(), &TSI);