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,
instead of disabling availability checking.
In this PR i worked on replacing the word accessor in diagnostics with more user-facing terminologies like setter, getter, didSet Observer, and members based on the context of the message.
in some messages i didn't need to pass DescriptiveDeclKind instead i just changed the text copy itself.
i also updated tests, so you might find it easier to check my changes this way.
Please let me know if there's something i should've done in a better way, and request changes if needed. !
i can squash my commits after reviewing getting the PR Reviewed, just to make it easier to be checked commit by commit
Resolves#55887
1. Removes gating on -enable-experimental-concurrency.
2. Updates eff. prop tests to remove experimental flag,
and also adjusts some tests slightly to avoid things
that are still behind that flag.
They're currently incompatible, but it should be
possible to enable this, with some care taken to
ensure that effectful wrappers are composed
correctly.
