Prohibit isolated conformances with Sendable(Metatype) constraints

[DougGregor]

Isolated conformances can only safely be used in a manner that will never escape out of the isolation domain to which they are restricted. There are two rules governing this:

1. The isolated conformance can only be referenced within the same isolation domain, e.g., a @MainActor conformance can only be referenced from @MainActor code.
2. The isolated conformance cannot escape out of the domain, e.g., by being used in conjunction with generic code that treats the type as Sendable or whose metatype is Sendable.

This pull request implements the second rule by prohibiting isolated conformances from being used to satisfy constraints on generic parameters that conform to either Sendable or SendableMetatype. There are two places we need this same check: when checking the wellformedness of substitution maps in general (e.g., for forming types), and within the constraint system. The first is mostly straightforward, although there are gaps in this implementation that involve associated conformances.

Within the constraint system, we introduce a new kind of conformance constraint, a "nonisolated conforms-to" constraint, which can only be satisfied by nonisolated conformances. Introduce this constraint instead of the normal conforms-to constraint whenever the subject type is a type parameter that has either a Sendable or SendableMetatype constraint, i.e., when the type or its values can escape the current isolation domain.

[DougGregor]

@swift-ci please smoke test