Fix an edge case in determining is a function has a prototype

Given the declaration:

  typedef void func_t(unsigned);
  __attribute__((noreturn)) func_t func;

we would incorrectly determine that `func` had no prototype because the
`noreturn` attribute would convert the underlying type directly into a
FunctionProtoType, but the declarator for `func` itself was not one for
a function with a prototype. This adds an additional check for when the
declarator is a type representation for a function with a prototype.