swift234

Sema: Implement DefaultWitnessChecker

Now that WitnessChecker is separate from ConformanceChecker, implement
a DefaultWitnessChecker subclass which performs default witness
resolution.

This populates the recently-added ProtocolDecl::DefaultWitnesses map.

Unlike ConformanceChecker, the DefaultWitnessChecker looks up the witness
in any protocol extensions of the protocol, matching the context archetypes
of the requirement against the witness.

For now, we infer default witnesses for all protocols, but don't do
anything with that information. An upcoming SILGen patch will start to
emit thunks and add tests.