c++: dependence of constrained memfn from current inst [PR105842]

Here we incorrectly deem the calls to func1, func2 and tmpl2 as
ambiguous ahead of time ultimately because we mishandle dependence
of a constrained member function from the current instantiation.

In type_dependent_expression_p, we already consider dependence of a
TEMPLATE_DECL's constraints (via uses_outer_template_parms), but
neglect to do the same for a FUNCTION_DECL (such as that for func1).

And in satisfy_declaration_constraints, we give up if _any_ template
argument is dependent, but for non-dependent member functions from
the current instantiation (such as func2 and tmpl2), we can and must
check constraints as long as the innermost arguments aren't dependent.

	PR c++/105842

gcc/cp/ChangeLog:

	* constraint.cc (satisfy_declaration_constraints): Refine early
	exit test for argument dependence.
	* cp-tree.h (uses_outer_template_parms_in_constraints): Declare.
	* pt.cc (template_class_depth): Handle TI_TEMPLATE being a
	FIELD_DECL.
	(usse_outer_template_parms): Factor out constraint dependence
	test into ...
	(uses_outer_template_parms_in_constraints): ... here.
	(type_dependent_expression_p): Use it for FUNCTION_DECL.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-memtmpl6.C: New test.

(cherry picked from commit f07778f6f92111aa0abfd0f669b148a0bda537a9)

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