Replicate the changes made by this commit
f2a0df309f78e1b5d7c5f81ada5110523644559e perfomed by Pietro on the
branch containing the development fixes for `revng-c`.
Handle the emission of variadic functions (such as the
`indirect_handler` helper function) both in regards if declaration and
definition.
Handle also `undef` values in calls to the `indirect_handler` function.