Specifically:
1. When we convert a function to nonisolated(nonsending), we need to
make sure that in the thunk we hop upon return since nonisolated(nonsending)
functions are assumed to preserve the caller's isolation.
2. When we convert a function from nonisolated(nonsending), we need to
make sure that in the thunk we hop onto the actor that we are passing in as the
isolated parameter of the nonisolated(nonsending) function. This ensures that
the nonisolated(nonsending) function can assume that it is already on its
isolated parameter's actor at function entry.
rdar://155905383
This ensures that when we generate the vtable thunk for a
nonisolated(nonsending) override (or vis-a-versa), we get the ABI correct. I
also added tests for all of the relevant cases for vtables that we check for
protocols.
rdar://151394209
(cherry picked from commit ef23f97071)
