This is a crucial fix without which we can crash on some distributed
protocol declarations with @Resolvable. We cannot "just" use a String to
represent the "fake base" of the thunks, and must instead find the
$Target macro generated type and use it as the base of the thunk's
mangling.
Calls are made in such way that record for the protocol requirement:
`$s4main28GreeterDefinedSystemProtocolP5greetSSyYaKFTEHF` points at
`$$s4main29$GreeterDefinedSystemProtocolC5greetSSyYaKFTE` which makes a
dispatch through the _apropriate_ witness table.
And the record for the $witness named e.g.
`$s4main29$GreeterDefinedSystemProtocolC5greetSSyYaKFTEHF` points to
`$s4main28GreeterDefinedSystemProtocolPAA11Distributed01_F9ActorStubRzrlE5greetSSyYaKFTE`
which is an extension method: `distributed thunk (extension in main):main.GreeterDefinedSystemProtocol< where A: Distributed._DistributedActorStub>.greet() async throws -> Swift.String`,
this very specific design allows us to call the "right method" on the
recieving end of a remote call where we do not know the recipient type.
