[ItaniumMangle] Mangle dependent __underlying_type correctly

We attempted to use the UnaryTransformType's UnderlyingType instead of
it's BaseType.  This is not correct for dependent UnaryTransformType
because the have no underlying type.

This fixes PR28045.

llvm-svn: 272079
diff --git a/clang/lib/AST/ItaniumMangle.cpp b/clang/lib/AST/ItaniumMangle.cpp
index 1d40d42..bb4039e 100644
--- a/clang/lib/AST/ItaniumMangle.cpp
+++ b/clang/lib/AST/ItaniumMangle.cpp
@@ -2736,7 +2736,7 @@
     }
   }
 
-  mangleType(T->getUnderlyingType());
+  mangleType(T->getBaseType());
 }
 
 void CXXNameMangler::mangleType(const AutoType *T) {