PR c++/38172
	PR c++/54379
gcc/c-family/
	* c-lex.c (c_common_has_attribute): Handle nodiscard.
gcc/cp/
	* parser.c (cp_parser_std_attribute): Handle [[nodiscard]].
	* tree.c (handle_nodiscard_attribute): New.
	(cxx_attribute_table): Add [[nodiscard]].
	* cvt.c (cp_get_fndecl_from_callee, cp_get_callee_fndecl): New.
	(maybe_warn_nodiscard): New.
	(convert_to_void): Call it.

From-SVN: r235597