Add a testcase which uses an UnresolvedUsingTypenameDecl as the base of an NNS.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@184386 91177308-0d34-0410-b5e6-96231b3b80d8

test/SemaCXX/using-decl-templates.cpp

@@ -80,3 +80,8 @@ namespace PR10883 {
     void foo(const Container& current); // expected-error {{unknown type name 'Container'}}
   };
 }
+
+template<typename T> class UsingTypenameNNS {
+  using typename T::X;
+  typename X::X x;
+};