Implement C++17 [[maybe_unused]] attribute.

Implement C++17 [[maybe_unused]] attribute.

gcc/
	* attribs.c (register_scoped_attributes): Fix logic.
	* attribs.h: Declare register_scoped_attributes.
c-family/
	* c-common.c (handle_unused_attribute): Accept CONST_DECL.
	No longer static.
	* c-common.h: Declare it.
	* c-lex.c (c_common_has_attribute): Add maybe_unused.
cp/
	* tree.c (std_attribute_table): New.
	(init_tree): Register it.

From-SVN: r235414