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.
When a protocol witness thunk is formed to a serialised protocol
containing an `async` function, the async function pointer to the
conformance needs to be made public due to a SIL Verifier check failure
(reference to a non-fragile function from within a public fragile
function). Add a stop-gap solution of rolling an extra emission for a
private symbol as a public symbol to avoid the error (the underlying
issue has been open for ~6y and counting as of this commit).
This was identified by swift-package-manager (#64900).
Thanks to @DougGregor and @aschwaighofer for the discussion on this!
