Implement propagation rules for global actor constraints, which can come from:

* Enclosing extension or type
* Superclass of a class
* Overridden declaration
* Requirement witnessed by a declaration
* Storage declaration for an accessor

ActorIsolation is rendered as a descriptive phrase before an entity,
e.g, "actor-independent" or "global actor 'UIActor'-isolated" when
used in diagnostics.

…asses.

Both overriding declarations and subclasses must have the actor
isolation as their overridden declarations or superclasses,
respectively. Enforce this, ensuring that we're also doing the
appropriate substitutions.

Subclasses inherit the global actor from their superclass by default,
but it's okay to change it---it's just a default that can be
overridden on a per-member basis anyway.

…g them.