c++: auto(x) partial substitution [PR110025, PR114138]

In r12-6773-g09845ad7569bac we gave CTAD placeholders a level of 0 and
ensured we never replaced them via tsubst.  It turns out that autos
representing an explicit cast need the same treatment and for the same
reason: such autos appear in an expression context and so their level
gets easily messed up after partial substitution, leading to premature
replacement via an incidental tsubst instead of via do_auto_deduction.

This patch fixes this by extending the r12-6773 approach to auto(x).

	PR c++/110025
	PR c++/114138

gcc/cp/ChangeLog:

	* cp-tree.h (make_cast_auto): Declare.
	* parser.cc (cp_parser_functional_cast): If the type is an auto,
	replace it with a level-less one via make_cast_auto.
	* pt.cc (find_parameter_packs_r): Don't treat level-less auto
	as a type parameter pack.
	(tsubst) <case TEMPLATE_TYPE_PARM>: Generalize CTAD placeholder
	auto handling to all level-less autos.
	(make_cast_auto): Define.
	(do_auto_deduction): Handle replacement of a level-less auto.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp23/auto-fncast16.C: New test.
	* g++.dg/cpp23/auto-fncast17.C: New test.
	* g++.dg/cpp23/auto-fncast18.C: New test.

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