c++: relax ref-qual overloading rules for C++20 [PR98939]

c++: relax ref-qual overloading rules for C++20 [PR98939]

As explained in one of Tomasz's library papers P2438R2[1], C++20 allows
ref-qualified member overloads to coexist with non-ref-qualified ones,
but at the time of writing no compiler supported that.  This patch
implements this C++20 relaxation so that we can in turn implement
P2438R2 as originally intended -- without needing to change the
signature of the main string::substr overload.

[1]: https://wg21.link/P2438R2#_modifying_existing_const_overload

	PR c++/98939
	PR libstdc++/119745

gcc/cp/ChangeLog:

	* class.cc (object_parms_correspond): Allow differing
	FUNCTION_REF_QUALIFIED in C++20.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/ref-qual5.C: Expect no diagnostics in C++20.

Reviewed-by: Jason Merrill <jason@redhat.com>