FunctionSupport: wrap around bool to have a more semantic callback type

This changes the type of the function type-building callback from
(ArrayRef<Type>, ArrayRef<Type>, bool, string &) to (ArrayRef<Type>,
ArrayRef<Type>, VariadicFlag, String &) to make the intended use clear from the
callback signature alone.

Also rearrange type definitions in Parser.cpp to make them more sorted
alphabetically.

PiperOrigin-RevId: 262405851