c++: add fixed testcase [PR63378]

We accept this testcase since r12-4453-g79802c5dcc043a.

	PR c++/63378

gcc/testsuite/ChangeLog:

	* g++.dg/template/fnspec3.C: New test.

gcc/testsuite/g++.dg/template/fnspec3.C

+// PR c++/63378
+// { dg-do compile { target c++11 } }
+
+template<class T1, class S1>
+struct B { };
+
+template<class T1>
+struct A {
+private:
+  template<class T2, class S2>
+  static B<T2, S2> g();
+
+public:
+  template<class S2>
+  auto f() -> decltype(g<T1, S2>());
+};
+
+template<>
+template<>
+auto A<int>::f<float>() -> B<int, float>;