[region-isolation] Transferring results shouldn't have the following error emitted: "non-sendable type 'NonSendableKlass' returned by implicitly asynchronous call to main actor-isolated function cannot cross actor boundary"

[gottesmm]

In this PR, I am chopping down a larger PR to make it easier to get in. This does the following:

1. Transferring results shouldn't have the following error emitted: "non-sendable type 'NonSendableKlass' returned by implicitly asynchronous call to main actor-isolated function cannot cross actor boundary". Transferring makes this safe, so I taught Sema to suppress this. We still do not get the correct behavior for async let... but I am working on that in a separate PR.
2. I improved how we print transferring. This included putting in a missing space in the AST printing and printing transferring_result on AST types that have a transferring result. Finally, it fixes usesFeatureTransferringArgsAndResults so we properly trigger if a function decl uses that feature.
3. While doing other work in async let, I noticed that we did not handle the case of an async let without any captured parameters correctly (we would crash in a dyn_cast). I think we might have gotten lucky and gotten it to work without asserts, but with this fix, we represent the behavior explicitly and correctly.
4. Finally I noticed that the strongly_transferring tests did not have strict-concurrency enabled. I enabled them on the test s and added an assert that if region isolation is enabled, then strict concurrency must also be enabled.

rdar://123712033

[gottesmm]

@swift-ci smoke test

[gottesmm]

@swift-ci smoke test

[gottesmm]

@swift-ci smoke test

[gottesmm]

@swift-ci test windows platform

[gottesmm]

Windows failed b/c of something unrelated in Foundation