Fixtype of possible classifications of certain constructs.
This is a tagged union type, introduced by fty::deftagsum.
Under certain circumstances, the parser needs to figure out whether what comes next in the input is a parameter type list or a declarator: see the discussion in type-spec. However, this cannot be always disambiguated, and so we need a third outcome for this case. This fixtype is the result of classify-partys/declor/ambig.