c++: warning for dependent template members [PR70417]

Add a helpful warning message for when the user forgets to
include the "template" keyword after ., -> or :: when
accessing a member in a dependent context, where the member is a
template.

	PR c++/70417

gcc/c-family/ChangeLog:

	* c.opt: Added -Wmissing-template-keyword.

gcc/cp/ChangeLog:

	* parser.c (cp_parser_id_expression): Handle
	-Wmissing-template-keyword.
	(struct saved_token_sentinel): Add modes to control what happens
	on destruction.
	(cp_parser_statement): Adjust.
	(cp_parser_skip_entire_template_parameter_list): New function that
	skips an entire template parameter list.
	(cp_parser_require_end_of_template_parameter_list): Rename old
	cp_parser_skip_to_end_of_template_parameter_list.
	(cp_parser_skip_to_end_of_template_parameter_list): Refactor to be
	called from one of the above two functions.
	(cp_parser_lambda_declarator_opt)
	(cp_parser_explicit_template_declaration)
	(cp_parser_enclosed_template_argument_list): Adjust.

gcc/ChangeLog:

	* doc/invoke.texi: Documentation for Wmissing-template-keyword.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/variadic-mem_fn2.C: Catch warning about missing
	template keyword.
	* g++.dg/template/dependent-name17.C: New test.
	* g++.dg/template/dependent-name18.C: New test.

Co-authored-by: Jason Merrill <jason@redhat.com>