This removes the IsolatedConformances feature gate from SILGen for
dynamic casts, such that all dynamic casts that will not work with
isolated conformances are marked as such. Isolated conformances can
still only come into a program when part of it enables the feature
flag.
When performing a dynamic cast to an existential type that satisfies
(Metatype)Sendable, it is unsafe to allow isolated conformances of any
kind to satisfy protocol requirements for the existential. Identify
these cases and mark the corresponding cast instructions with a new flag,
`[prohibit_isolated_conformances]` that will be used to indicate to the
runtime that isolated conformances need to be rejected.
