Fixtype of integer suffixes [C:6.4.4.1] [C:A.1.5].
This is a tagged union type, introduced by fty::deftagsum.
This corresponds to integer-suffix in the grammar in [C]. This captures the four possibilities of (i) just an unsigned suffix, (ii) just a length suffix, (iii) an unsigned suffix followed by a length suffix, and (iv) a length suffix followed by an unsigned suffix.