[silgen] Use Builtin.ImplicitActor instead of Optional<any Actor> to represent the implicit isolated parameter.
NOTE: We are not performing any bitmasking at all now. This is so that we can
transition the code base/tests to expect Builtin.ImplicitActor instead
of Optional<any Actor>.
NOTE: The actual test changes are in the next commit. I did this to make it
easier to review the changes.
This should not have any user visible changes.
(cherry picked from commit 788abd0b96b055853470b73731f0ab3a5c40a85c)
swift
ce162afd - [silgen] Use Builtin.ImplicitActor instead of Optional<any Actor> to represent the implicit isolated parameter.
