Fixtype of integer constants [C:6.4.4.1] [C:A.1.5].
This is a product type introduced by fty::defprod.
This corresponds to integer-constant in the grammar in [C]. As mentioned in dec/oct/hex-const, our fixtypes are factored slightly differently. An integer constant consists of a decimal, octal, or hexadecimal constant, and of an optional integer suffix.