Completely mechanical changes:
- Explicit @objc in a few places
- Some imported APIs changed
- For the mix-and-match tests, just test version 4/5 instead of 3/4
This converts the instances of the pattern for which we have a proper
substitution in lit. This will make it easier to replace it
appropriately with Windows equivalents.
Deserializing a witness record in a conformance may fail if either of the requirement or witness changed name or type, most likely due to SDK modernization changes across Swift versions. When this happens, leave an opaque placeholder in the conformance to indicate that the witness exists but we don't get to see it. For expedience, right now this just witnesses the requirement to itself, so that code in the type checker or elsewhere that tries to ad-hoc devirtualize references to the requirement just gets the requirement back. Arguably, we shouldn't include the witness at all in imported conformances, since they should be an implementation detail, but that's a bigger, riskier change. This patch as is should be enough to address rdar://problem/31185053.
