This means that:
1. In test cases where minimal is the default (swift 5 without
-warn-concurrency), I added RUN lines for targeted, complete, and complete +
sns.
2. In test cases where complete is the default (swift 6, -warn-concurrency,
specified complete with -strict-concurrency), I added a send non-sendable run
line.
In each of these cases, I added additional expected-* lines as appropriate so
the tests can compile in each mode successfully.
This isn't implemented. It would never make sense for escaping captures of
`async let` values, because doing so would either involve an implicit awaiting
of the value on capture, or else stretching the lifetime of the task past its
scope. In nonescaping contexts, and in nested `async let` expressions, it might
be implementable in the future. rdar://80043610
