[Serialization] (De-)serialize witnesses completely.

[DougGregor]

The witnesses in a NormalProtocolConformance have never been
completely serialized, because their substitutions involved a weird
mix of archetypes that blew up the deserialization code. So, only the
witness declarations themselves got serialized. Many clients (the type
checker, SourceKit, etc.) didn't need the extra information, but some
clients (e.g., the SIL optimizers) would end up recomputing this
information. Ick.

Now, serialize the complete Witness structure along with the AST,
including information about the synthetic environment, complete
substitutions, etc. This should obsolete some redundant code paths in
the SIL optimization infrastructure.

This (de-)serialization code takes a new-ish approach to serializing
the synthetic environment in that it avoids serializing any
archetypes. Rather, it maps everything back to interface types during
serialization, and deserialization forms a new generic environment
(with new archetypes!) on-the-fly, mapping deserialized types back
into that environment (and to those archetypes). This way, we don't
have to maintain identity of archetypes in the deserialization code,
and might get some better re-use of the archetypes.

More of rdar://problem/24079818.

[DougGregor]

@swift-ci please smoke test and merge

[DougGregor]

@swift-ci please smoke test Linux