Fixtype of function declarators [C:6.7.6].
This is a tagged union type, introduced by fty::deftagsum.
For now we only model function declarators [C:6.7.6.3]
consisting of an identifier as the direct declarator
and a (parenthesized) list of parameter declarations,
preceded by zero or more pointer designations (i.e.
This is somewhat similar to obj-declor, except that there is an identifier and a list of parameters instead of just an identifier (for the base case of the fixtype), and except that there is no array designation possible. The latter is because functions cannot return array types [C:6.7.6.3/1].