Don't set allowMissing to true when checking conformance of a superclass requirement
to a protocol, since this prevents us from being able to express something like
'T : C, T : Sendable' to mean 'T can be any subclass of C which is also Sendable'.
Fixes rdar://problem/91530343.
A recent change to `Sendable` conformance handling resulted in subclasses of global-actor-confined classes being rejected if they explicitly declared a conformance to `Sendable`.
This behavior is technically correct because actor-isolated types are implicitly `Sendable`, but the source compatibility regression was not desirable. We are also considering requiring subclasses to explicitly repeat their superclass's `Sendable` conformance, so it makes sense to allow these redundant conformances in the general case to ease that potential transition.
Fixes rdar://88700507.
When a non-isolated requirement is witnessed by an actor-isolated
witness, we are crossing into the actor. Ensure that we perform
Sendable checking across the actor boundary here.
Fixes the rest of rdar://80424675.
