PR c++/91338 - Implement P1161R3: Deprecate a[b,c].

PR c++/91338 - Implement P1161R3: Deprecate a[b,c].

	* c-opts.c (c_common_post_options): Enable -Wcomma-subscript by
	default for C++2a, unless -Wno-deprecated.
	* c.opt (Wcomma-subscript): New warning.

	* parser.c (cp_parser_postfix_open_square_expression): Warn about uses
	of a comma operator within a subscripting expression.
	(cp_parser_skip_to_closing_square_bracket_1): New function, made out
	of...
	(cp_parser_skip_to_closing_square_bracket): ...this.

	* doc/invoke.texi: Document -Wcomma-subscript.

	* g++.dg/cpp2a/comma1.C: New test.
	* g++.dg/cpp2a/comma2.C: New test.
	* g++.dg/cpp2a/comma3.C: New test.
	* g++.dg/cpp2a/comma4.C: New test.

From-SVN: r274121