c++: Add fixed test [PR83596]

This was fixed by r269078.

	PR c++/83596

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1z/nontype5.C: New test.

gcc/testsuite/g++.dg/cpp1z/nontype5.C

+// PR c++/83596
+// { dg-do compile { target c++17 } }
+
+struct X { int x; int y; };
+template <int X::* mp> int get(X& x) { return x.*mp; }
+constexpr int X::* getMP() { return &X::y; }
+constexpr int X::* mptr = getMP();
+int test() {
+    X x{1, 2};
+    return get<mptr>(x);
+}