Implement C++17 constexpr lambda.

Implement C++17 constexpr lambda.

gcc/c-family/
	* c-cppbuiltin.c (c_cpp_builtins): Update __cpp_constexpr for
	C++17 constexpr lambdas.
gcc/cp/
	* class.c (finalize_literal_type_property): Handle lambdas.
	* constexpr.c (is_valid_constexpr_fn): Likewise.  No longer static.
	(explain_invalid_constexpr_fn, cxx_eval_call_expression): Handle
	lambdas.
	(cxx_eval_constant_expression): Handle capture proxy.
	(var_in_constexpr_fn): Don't check for C++14.
	(var_in_maybe_constexpr_fn): New.
	(potential_constant_expression_1): Use it.  Check DECL_EXPR for
	declarations not allowed in constexpr function.
	* decl.c (make_rtl_for_nonlocal_decl): Use var_in_maybe_constexpr_fn.
	(finish_function): Set DECL_DECLARED_CONSTEXPR_P on lambda members.
	* lambda.c (begin_lambda_type): Set CLASSTYPE_LITERAL_P.
	(maybe_add_lambda_conv_op): Clear thunk CALL_EXPR location.
	(lambda_static_thunk_p): New.
	* parser.c (cp_keyword_starts_decl_specifier_p): Add RID_CONSTEXPR.
	(CP_PARSER_FLAGS_ONLY_MUTABLE_OR_CONSTEXPR): New enumerator.
	(cp_parser_decl_specifier_seq): Handle it.
	(cp_parser_lambda_declarator_opt): Use cp_parser_decl_specifier_seq.
	* pt.c (instantiate_class_template_1): Set CLASSTYPE_LITERAL_P.
	(tsubst_copy_and_build) [CALL_EXPR]: Propagate CALL_FROM_THUNK_P.
	* error.c (dump_function_decl): Check TFF_NO_TEMPLATE_BINDINGS.
	(dump_expr) [FUNCTION_DECL]: Pass it.

From-SVN: r239268