A witness table instantiation function can end up referring to
conditional requirements to, e.g., fill in base witness tables. Store
the conditional requirements first, and also bind the witness tables
for conditional requirements within the local scope so they can be
used directly. Fixes SR-6478.
When emitting a witness table accessor (e.g., an associated type metadata
or associated type conformance accessor) for a conditional conformance,
bind the conditional requirements so we can refer to them within that
accessor. Witness methods get this behavior already through the SelfWitnessTable parameter, but everything the witness table needs it.
Fixes most of SR-6478.
