c++: add fixed test [PR119378]

Fixed by r15-123 (specifically the change to set processing_template_decl
when tsubsting UNBOUND_CLASS_TEMPLATE).

	PR c++/119378

gcc/testsuite/ChangeLog:

	* g++.dg/template/friend85.C: New test.

gcc/testsuite/g++.dg/template/friend85.C

@@ -0,0 +1,16 @@
+// PR c++/119378
+
+template<int N>
+struct A {
+  template<class T>
+  struct B;
+};
+
+template<class U>
+struct C {
+  template<int N>
+  template<class T>
+  friend class A<N>::B;
+};
+
+template struct C<int>;