c++: add "'requires' only available with ..." note

This adds a note suggesting to enable concepts whenever 'requires' is parsed as
an invalid type name with concepts disabled.

gcc/cp/ChangeLog:

	* parser.c (cp_parser_diagnose_invalid_type_name): Suggest enabling
	concepts if the invalid identifier is 'requires'.

gcc/testsuite/ChangeLog:

	* g++.dg/concepts/diagnostic11.C: New test.