Add test for C++ [temp.friend]p8, which bans partial specializations from being friends

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@122335 91177308-0d34-0410-b5e6-96231b3b80d8

test/CXX/temp/temp.decls/temp.friend/p8.cpp

+// RUN: %clang_cc1 -fsyntax-only -verify %s
+template<class T> class A { }; 
+
+class X {
+  template<class T> friend class A<T*>; // expected-error{{partial specialization cannot be declared as a friend}}
+};