Reparenting: sign the conformance descriptor
The protocol conformance descriptor passed to swift_getWitnessTable
needs to be signed on arm64e. When reparenting a protocol P with
new parent Q, we emit a small "default conformance accessor" function
that instantiates the type's witness table for Q using its witness
table for P, at runtime. This accessor is only used if the type
conforming to P comes from a module that hasn't been rebuilt since
the reparenting happened.
resolves rdar://171661924
