Fix two bugs with the isolation of defer bodies.
The first bug is that we weren't computing isolation correctly for
nested defers. This is an unlikely pattern of code, but it's good to fix.
The second bug is that getActorIsolationOfContext was looking through
defers, but getActorIsolation itself was not. This was causing defer
bodies to be emitted in SILGen without an isolation parameter, which
meant that #isolation could not possibly provide the right value. Fixing
this involves teaching SILGen that non-async functions can have
nonisolated(nonsending) isolation, but that's relatively straightforward.
This commit doesn't fix #isolation or adequately test SILGen, but that'll
be handled in a follow-up.
