c++: normalization of non-templated return-type-req [PR100946]

Here the satisfaction cache is conflating the satisfaction value of the
two return-type-requirements because the corresponding constrained
'auto's have level 2, but they capture an empty current_template_parms.
This ultimately causes the satisfaction cache to think the type
constraint doesn't depend on the deduced type of the expression.

When normalizing the constraints on an 'auto', the assumption made by
normalize_placeholder_type_constraints is that the level of the 'auto'
is one greater than the depth of the captured current_template_parms, an
assumption which is not holding here.  So this patch just makes n_p_t_c
adjust the normalization context appropriately in this situation.

	PR c++/100946

gcc/cp/ChangeLog:

	* constraint.cc (normalize_placeholder_type_constraints): When
	normalizing a non-templated return-type-requirement, add a dummy
	level to initial_parms.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-return-req3.C: New test.