Implement P1814R0, CTAD for alias templates.

Implement P1814R0, CTAD for alias templates.

This patch implements C++20 class template argument deduction for alias
templates, which works by a moderately arcane transformation of the
deduction guides for the underlying class template.  When implementing it,
it seemed that I could simplify the rules in the draft a bit and get
essentially the same effect; I'll be emailing the committee to that effect
soon.

gcc/cp/
	* pt.c (rewrite_tparm_list): Factor out of build_deduction_guide.
	(maybe_aggr_guide): Check for copy-init here.
	(alias_ctad_tweaks, deduction_guides_for): New.
	(ctor_deduction_guides_for): Factor out of do_class_deduction.
	(ctad_template_p): New.
	* parser.c (cp_parser_simple_type_specifier): Use it.
	* constraint.cc (append_constraint): New.
gcc/c-family/
	* c-cppbuiltin.c (c_cpp_builtins): Update __cpp_deduction_guides.

From-SVN: r278786