c++: Improve CTAD for aggregates [PR93976]

c++: Improve CTAD for aggregates [PR93976]

P2082R1 adjusted the rules for class template argument deduction for an
aggregate to better handle arrays and pack expansions.

gcc/cp/ChangeLog:

	PR c++/93976
	Implement C++20 P2082R1, Fixing CTAD for aggregates.
	* cp-tree.h (TPARMS_PRIMARY_TEMPLATE): Split out from...
	(DECL_PRIMARY_TEMPLATE): ...here.
	(builtin_guide_p): Declare.
	* decl.c (reshape_init_class): Handle bases of a template.
	(reshape_init_r): An array with dependent bound takes a single
	initializer.
	* pt.c (tsubst_default_argument): Shortcut {}.
	(unify_pack_expansion): Allow omitted arguments to trailing pack.
	(builtin_guide_p): New.
	(collect_ctor_idx_types): Give a trailing pack a {} default
	argument.  Handle arrays better.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/class-deduction-aggr3.C: New test.
	* g++.dg/cpp2a/class-deduction-aggr4.C: New test.