The concurrency runtime now deploys back to macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, which corresponds to the 5.1 release of the stdlib.
Adjust macro usages accordingly.
The validation-test suite was incorrectly being run without the
concurrency feature enabled, so the tests were disabled. Now that
they're running, they need fixed availability annotations.
Previously, when looking up a protocol method, the witness table was
always exepcted to be the final argument passed to the function.
That is true for sync protocol witnesses but not for async witnesses.
In the async case, the witness table is embedded in the async context.
Here, the witness table is dug out of the async context.
rdar://problem/71491604
