PR libstdc++/80251
c-family/
	* c-common.h (enum rid): Add RID_IS_AGGREGATE.
	* c-common.c (c_common_reswords): Add __is_aggregate trait.
cp/
	* cp-tree.h (enum cp_trait_kind): Add CPTK_IS_AGGREGATE.
	* cxx-pretty-print.c (pp_cxx_trait_expression): Handle
	CPTK_IS_AGGREGATE.
	* semantics.c (trait_expr_value): Handle CPTK_IS_AGGREGATE.
	Remove extraneous parens.
	(finish_trait_expr): Handle CPTK_IS_AGGREGATE.
	* parser.c (cp_parser_primary_expression): Handle RID_IS_AGGREGATE.
	(cp_parser_trait_expr): Likewise.
testsuite/
	* g++.dg/ext/is_aggregate.C: New test.

From-SVN: r246609