Add missing test case that wasn't uploaded

Author: ilovepi

clang/test/Sema/PR166843.cpp

@@ -0,0 +1,7 @@
+// RUN: %clang_cc1 -fsyntax-only %s -verify
+namespace a {
+template <class b>
+void c() {
+  ((::c::)); // expected-error {{expected unqualified-id}}
+}
+}