c++: Fix CTAD of single-element initializer list [PR99103]

c++: Fix CTAD of single-element initializer list [PR99103]

When determining whether to rule out initializer-list constructors
during CTAD with a single-element initializer list (as per P0702), the
element type's cv-qualifiers should be irrelevant.  This patch fixes
this by making is_spec_or_derived strip cv-qualifiers from the supplied
expression type.

In passing, I noticed in maybe_aggr_guide we were calling
is_spec_or_derived with swapped arguments.  This led us to prefer the
aggregate deduction candidate over copying deduction in the second
testcase below with -std=c++20.

gcc/cp/ChangeLog:

	PR c++/99103
	* pt.c (is_spec_or_derived): Drop cv-qualifiers from 'etype'.
	(maybe_aggr_guide): Fix order of arguments to is_spec_or_derived.

gcc/testsuite/ChangeLog:

	PR c++/99103
	* g++.dg/cpp1z/class-deduction79.C: New test.
	* g++.dg/cpp1z/class-deduction80.C: New test.