c++: Dependent using enum [PR97874]

c++: Dependent using enum [PR97874]

The handling of dependent scopes and unsuitable scopes in lookup_using_decl
was a bit convoluted; I tweaked it for a while and then eventually
reorganized much of the function to hopefully be clearer.  Along the way I
noticed a couple of ways we were mishandling inherited constructors.

The local binding for a dependent using is the USING_DECL.

Implement instantiation of a dependent USING_DECL at function scope.

gcc/cp/ChangeLog:

	PR c++/97874
	* name-lookup.c (lookup_using_decl): Clean up handling
	of dependency and inherited constructors.
	(finish_nonmember_using_decl): Handle DECL_DEPENDENT_P.
	* pt.c (tsubst_expr): Handle DECL_DEPENDENT_P.

gcc/testsuite/ChangeLog:

	PR c++/97874
	* g++.dg/lookup/using4.C: No error in C++20.
	* g++.dg/cpp0x/decltype37.C: Adjust message.
	* g++.dg/template/crash75.C: Adjust message.
	* g++.dg/template/crash76.C: Adjust message.
	* g++.dg/cpp0x/inh-ctor36.C: New test.
	* g++.dg/cpp1z/inh-ctor39.C: New test.
	* g++.dg/cpp2a/using-enum-7.C: New test.