Add a test to check that attempting to `Task.sleep(until:)` passing a
time before the start of the Dispatch clocks works and doesn't wait
forever.
rdar://148899609
In the Dispatch implementation, clamp the delay to INT64_MAX. Swift's nanoseconds value is unsigned, but we ultimately use it with dispatch_time, which takes a signed int64_t. Extremely large values get interpreted as negative, which results in not sleeping.
INT64_MAX nanoseconds is about 292 years, so it should be difficult to notice a practical effect from sleeping for less time than requested due to the clamping.
rdar://143278824
Instead, 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.
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.
Rather than blanket-disabling concurrency tests when we aren't using a
just-built concurrency library, enable them whenever we have a
suitable concurrency runtime, either just-built, in the OS, or via the
back-deployment libraries.
This allows programs to target older OSes while using Concurrency behind an availability check. When targeting older OSes, the symbols are weak-linked and the compiler will require the use of Concurrency features to be guarded by an availability check.
rdar://75850003
