re PR libstdc++/51365 (cannot use final empty class in std::tuple)

re PR libstdc++/51365 (cannot use final empty class in std::tuple)

c-family:
	PR libstdc++/51365
	* c-common.c (RID_IS_FINAL): Add.
	* c-common.h (RID_IS_FINAL): Add.
cp:
	PR libstdc++/51365
	* cp-tree.h (CPTK_IS_FINAL): Add.
	* parser.c (cp_parser_translation_unit): Handle RID_IS_FINAL.
	(cp_parser_primary_expression, cp_parser_trait_expr): Likewise.
	* semantics.c (trait_expr_value, finish_trait_expr): Handle
	CPTK_IS_FINAL.
	* cxx-pretty-print.c (pp_cxx_trait_expression): Likewise.
testsuite:
	PR libstdc++/51365
	* g++.dg/ext/is_final.C: New.

From-SVN: r182360