gengtype-lex.l: Distinguish unions from structures in the token type.

gengtype-lex.l: Distinguish unions from structures in the token type.

	* gengtype-lex.l: Distinguish unions from structures in the
	token type.  Don't call find_structure; return the tag as a string.
	* gengtype-yacc.y: Add new token types ENT_TYPEDEF_UNION and ENT_UNION.
	Type of these, ENT_TYPEDEF_STRUCT, and ENT_STRUCT is string.
	Reorganize typedef_struct production accordingly.
	Use create_nested_ptr_option.
	* gengtype.c (create_nested_ptr_option): New function.
	* gengtype.h: Declare it.

From-SVN: r123232