Implement P1816R0, class template argument deduction for aggregates.

Implement P1816R0, class template argument deduction for aggregates.

Rather than reimplement brace elision here, we call reshape_init and then
discard the result.  We needed to set CLASSTYPE_NON_AGGREGATE a bit more in
this patch, since outside a template it's set in check_bases_and_members.

	* pt.c (maybe_aggr_guide, collect_ctor_idx_types): New.
	(is_spec_or_derived): Split out from do_class_deduction.
	(build_deduction_guide): Handle aggregate guide.
	* class.c (finish_struct): Set CLASSTYPE_NON_AGGREGATE in a
	template.
	* cp-tree.h (CP_AGGREGATE_TYPE_P): An incomplete class is not an
	aggregate.

From-SVN: r278298