libstdc++: Add test for type traits not having friend access

This ensures that the std::is_assignable and std::is_assignable_v
traits are evaluated "in a context unrelated" to the argument types.

libstdc++-v3/ChangeLog:

	* testsuite/20_util/is_assignable/requirements/access.cc:
	New test.

libstdc++-v3/testsuite/20_util/is_assignable/requirements/access.cc

@@ -0,0 +1,22 @@
+// { dg-do compile { target c++11 } }
+
+#include <type_traits>
+
+class S {
+  operator int();
+  friend void g(); // #1
+};
+
+void
+g()
+{
+  int i = 0;
+  S s;
+  i = s; // this works, because we're inside a friend.
+
+  // But the traits are evaluated in "a context unrelated to either type".
+  static_assert( ! std::is_assignable<int&, S>::value, "unfriendly");
+#if __cplusplus >= 201703L
+  static_assert( ! std::is_assignable_v<int&, S>, "unfriendly");
+#endif
+}