...and remove the option. This is ~technically~ CLI-breaking because
Swift 5 shipped this as a hidden driver option, but it wouldn't have
/done/ anything in Swift 5, so I think it's okay to remove.
Note that if a parseable interface (.swiftinterface) and a binary
interface (.swiftmodule) are both present, the binary one will still
be preferred. This just /allows/ parseable interfaces to be used.
rdar://problem/36885834
This would break resilience, which makes a distinction between "SIL
only inside the module, which is inside the resilience domain" and
"SIL we're going to serialize, which is outside the resilience
domain".
