Remove the heuristic that escaping closures cannot be actor-isolated.
This is in line with the long term plan for actor isolation, but opens
up some holes in the short term because Sendable closures are not enforced
throughout the Swift ecosystem.
One significant effect of this change is that we will now, statically,
fail to detect many cases where actor isolation can be broken.
Closures often end up being implicitly actor-isolated. Allow them to opt
out with `@actorIndependent`, as as other declarations can opt out of
actor isolation.
This patch updates the `actor class` spelling to `actor` in almost all
of the tests. There are places where I verify that we sanely handle
`actor` as an attribute though. These include:
- test/decl/class/actor/basic.swift
- test/decl/protocol/special/Actor.swift
- test/SourceKit/CursorInfo/cursor_info_concurrency.swift
- test/attr/attr_objc_async.swift
- test/ModuleInterface/actor_protocol.swift
Compute the actor isolation for every closure, noting whether it is
part of an actor instance (and which 'self' variable describes the
instance), global actor, or independent of any actor. This information
is required for propertly generating `hop_to_executor` instructions in
SIL.
Fixes rdar://71126554.
