If a protocol inherits from a protocol that can't be loaded, drop it
entirely. Similarly, if it has requirements that reference types in
other modules that can't be loaded, drop the protocol entirely---at
least for now, we don't want to deal with a protocol that exists but
has the wrong requirement signature. That "in other modules" isn't
perfect, but it avoids cases where two protocols depend on each other.
Unfortunately, it means the compiler may still get into exactly the
situation above if a protocol depends on another protocol in the same
module, and /that/ protocol can't be loaded for some other reason. But
it's progress.
This comes up when referencing implementation-only-imported protocols
from non-public protocols, but is also just general deserialization
recovery goodness.
rdar://problem/52141347
