cobol: Allow larger REDEFINES for -dialect ibm.

ISO prohibits B REDEFINES A if B is larger than A, unless A is 01
level and not external.  IBM and others do not impose the 01 restriction.

Also simplify and correct how -preprocess parses its argument.

gcc/cobol/ChangeLog:

	* cbldiag.h (enum cbl_diag_id_t): Define IsoRedefinesGrow.
	* cobol1.cc (cobol_langhook_handle_option): Handle Wredefines-grow.
	* lang-specs.h: Same.
	* lang.opt: Same.
	* lexio.cc (preprocess_filter_add): Use xstrdup, not std::vector.
	* messages.cc: Quell Wredefines-grow with -dialect ibm.
	* parse_ante.h (valid_redefine): Conditional message.