c-parse.in (struct_head, [...]): New nonterminals.

c-parse.in (struct_head, [...]): New nonterminals.

        * c-parse.in (struct_head, union_head, enum_head): New nonterminals.
        (structsp): Use them.  Update files generated from c-parse.in.
        * extend.texi (Type Attributes): Document it.

From-SVN: r20971