[Distributed] Fix `_executeDistributedFunction` resume and result buffer (#40814)
* [Distributed] Adjust interface of `swift_distributed_execute_target`
Since this is a special function, `calleeContext` doesn't point to
a direct parent but instead both parent context (uninitialized)
and resume function are passed as last arguments which means that
`callContext` has to act as an intermediate context in call to accessor.
* [Distributed] Drop optionality from result buffer in `_executeDistributedTarget`
`RawPointer?` is lowered into a two arguments since it's a struct,
to make it easy let's just allocate an empty pointer for `Void` result.
* [Distributed] NFC: Update _remoteCall test-case to check multiple different result types