allow move-only types to conform to `Sendable`
Part of the reason why we do not want to
permit conformance to protocols for
move-only types is that they're
fundamentally wrong: all existing protocols
assume the type is copyable, so we'd be
allowing people to write conformances to
things that are not actually true.
The other aspect of it is that we may
need to change the runtime representation
for conformance descriptors of move-only
types. So we can't have these conformances
leaving residue at runtime. Luckily, that
means marker protocols would be OK,
since they leave no residue at runtime.
So for, now we're going to specifically
permit move-only types to conform to the
marker protocol `Sendable` since it's needed
for move-only types to work with concurrency.
All of the existing rules about mixing
move-only types with generics still apply.
That means you still can't turn it into the
existential `any Sendable` in any way at all,
despite it conforming. The purpose of the
conformance is purely to allow the concrete
instances to cross actor isolation boundaries
if it is actually `Sendable`.
resolves rdar://104987062
