swift234

[Serialization] Allow witness conformances to be "incomplete" when writing.

This just means we haven't deserialized them properly yet. As soon as we
try to /do/ something with the conformance, we'd pull in what we need, but
for reserializing (i.e. module-merging) we can generally just keep going.

<rdar://problem/17251682>

Swift SVN r19540