PR c++/91391 - bogus -Wcomma-subscript warning.

PR c++/91391 - bogus -Wcomma-subscript warning.

	* parser.c (cp_parser_postfix_open_square_expression): Don't warn about
	a deprecated comma here.  Pass warn_comma_subscript down to
	cp_parser_expression.
	(cp_parser_expression): New bool parameter.  Warn about uses of a comma
	operator within a subscripting expression.
	(cp_parser_skip_to_closing_square_bracket): Revert to pre-r274121 state.
	(cp_parser_skip_to_closing_square_bracket_1): Remove.

	* g++.dg/cpp2a/comma5.C: New test.

Co-Authored-By: Marek Polacek <polacek@redhat.com>
From-SVN: r274483