PR c++/92590 - wrong handling of inherited default ctor.

PR c++/92590 - wrong handling of inherited default ctor.

I thought my earlier fix for 91930 was an obvious bug fix, but apparently an
inherited constructor does not count as user-declared.  So this patch
reverts that change and the other follow-on patches, and fixes 91930
differently, by not letting the inherited default constructor hide the
implicitly-declared default constructor.

	* class.c (add_method): A constrained inherited ctor doesn't hide an
	implicit derived ctor.

	Revert:
	PR c++/91930 - ICE with constrained inherited default ctor.
	* name-lookup.c (do_class_using_decl): Set TYPE_HAS_USER_CONSTRUCTOR
	for inherited constructor.
	PR c++/92552 - ICE with inherited constrained default ctor.
	* pt.c (instantiate_class_template_1): Copy
	TYPE_HAS_USER_CONSTRUCTOR.
	PR c++/92594 - ICE with inherited trivial default ctor.
	* method.c (trivial_fn_p): Treat an inherited default constructor
	like a normal default constructor.