swift234

[serialization] Write substitution conformances as references when possible.

More importantly, when writing substitution conformances /within the same
module/, use an "incomplete" form of the NormalProtocolConformance layout
that doesn't include any of the substitutions or defaulted definitions.
This avoids a serialization cycle when the witness for a protocol itself
ends up conforming to the protocol.

I couldn't come up with a reduced test cases, but both cases filed by Dave
now work.

<rdar://problem/16468715>

Swift SVN r15912