[swift] Parse Swift function comment into each arg/return type

From [orchidhq/Orchid #343](https://github.com/orchidhq/Orchid/issues/343), parse comment text from a function into each arg. Also, make the return type null if the function returns void.

Start by locating the Swift parser responsible for function comments and inspect how argument and return types are currently represented. Use the linked Orchid issue for context, then verify that each argument receives its comment text and that void returns produce a null return type.

Written by the indexing model from the issue text.