Instead, use the `%target-swift-5.1-abi-triple` substitution to compile the tests
for deployment to the minimum OS versions required for use of _Concurrency APIs.
Rather than blanket-disabling concurrency tests when we aren't using a
just-built concurrency library, enable them whenever we have a
suitable concurrency runtime, either just-built, in the OS, or via the
back-deployment libraries.
A uniqueness rule for 'self' is enforced throughout
the following types of actor initializers:
1. synchronous
2. global-actor isolated
This means that 'self' cannot be used for anything,
even after 'self' is fully initialized, other than
for access to stored properties. Method calls are
considered escaping uses of 'self', since it is passed
implicitly to the method of computed property when invoked.
Also, an actor's convenience init now treats self as
nonisolated.
This provides a way to perform follow-up work after
self is fully initialized, when creating the
actor from a synchronous context.
Resolves rdar://78790369
