[move-only] Do not try to capture self as an immutable box.
The problem here is that the logic was conditionalized on all noncopyable
parameters that are borrowed as having the ValueOwnership::Shared flag set. This
is only true for user written parameters. Implicit noncopyable parameters like
self do not have ValueOwnership::Shared set upon them. We could potentially do
that in Sema, but Sema does not know what the proper convention of self is since
that information is in TypeLowering today.
With that in mind, conditionalize the logic here so we do the right thing.
rdar://112547982
