Fixtype of floating constants [C:6.4.4.2] [C:A.1.5].
This is a tagged union type, introduced by fty::deftagsum.
This corresponds to floating-constant in the grammar in [C]. As mentioned in dec-core-fconst and hex-core-fconst, we factor things a bit differently in our fixtypes, so here we add the prefixes and suffixes as appropriate.