c++: inherited CTAD fixes [PR116276]

c++: inherited CTAD fixes [PR116276]

This implements the overlooked inherited vs non-inherited guide
tiebreaker from P2582R1.  This requires tracking inherited-ness of a
guide, for which it seems natural to reuse the lang_decl_fn::context
field which for a constructor tracks its inherited-ness.

This patch also works around CLASSTYPE_CONSTRUCTORS not reliably
returning all inherited constructors (due to some using-decl handling
quirks in in push_class_level_binding) by iterating over TYPE_FIELDS
instead.

This patch also makes us recognize another written form of inherited
constructor, 'using Base<T>::Base::Base' whose USING_DECL_SCOPE is a
TYPENAME_TYPE.

	PR c++/116276

gcc/cp/ChangeLog:

	* call.cc (joust): Implement P2582R1 inherited vs non-inherited
	guide tiebreaker.
	* cp-tree.h (lang_decl_fn::context): Document usage in
	deduction_guide_p FUNCTION_DECLs.
	(inherited_guide_p): Declare.
	* pt.cc (inherited_guide_p): Define.
	(set_inherited_guide_context): Define.
	(alias_ctad_tweaks): Use set_inherited_guide_context.
	(inherited_ctad_tweaks): Recognize some inherited constructors
	whose scope is a TYPENAME_TYPE.
	(ctor_deduction_guides_for): For C++23 inherited CTAD, iterate
	over TYPE_FIELDS instead of CLASSTYPE_CONSTRUCTORS to recognize
	all inherited constructors.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp23/class-deduction-inherited4.C: Remove an xfail.
	* g++.dg/cpp23/class-deduction-inherited5.C: New test.
	* g++.dg/cpp23/class-deduction-inherited6.C: New test.

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