* c-common.c (decl_attributes): Allow applying attribute `unused'
        on a LABEL_DECL.
        * c-parse.in (label): Parse attributes after a label, and call
        `decl_attributes' to handle them.
        * gansidecl.h (ATTRIBUTE_UNUSED_LABEL): Define.
        * genrecog.c (OUTPUT_LABEL, write_tree_1, write_tree): When
        generating labels, mark them with ATTRIBUTE_UNUSED_LABEL.
        * invoke.texi: Note that labels can be marked `unused'.

From-SVN: r24478