c++: ICE on unviable/ambiguous constrained dtors [PR96745]

c++: ICE on unviable/ambiguous constrained dtors [PR96745]

Here we're crashing from check_bases_and_members due to
CLASSTYPE_DESTRUCTOR being an OVERLOAD which, due to the pruning
performed by add_method, should only happen if there is no viable
destructor or the destructor is ambiguous because of unsatisfied
or ambiguous constraints.

This patch fixes this by making check_bases_and_members naturally handle
CLASSTYPE_DESTRUCTOR being an OVERLOAD.  It's then convenient to prune
the OVERLOAD after effectively diagnosing the overload resolution
failure in check_methods.

	PR c++/96745

gcc/cp/ChangeLog:

	* class.cc (check_methods): Diagnose an unviable OVERLOAD
	set for CLASSTYPE_DESTRUCTOR differently from an ambiguous one.
	Then prune the OVERLOAD to a single function.
	(check_bases_and_members): Handle CLASSTYPE_DESTRUCTOR being
	an OVERLOAD when calling deduce_noexcept_on_destructor.
	Document why it has to be called before check_methods.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-dtor1.C: New test.