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
The test has been flaky for the last couple of weeks. It almost always
fails, but from time to time the Windows VS2017 CI machine passes the
test.
Mark it as unsupported (since XFAIL will create random failures when the
test pass), to avoid noise in the CI machine.
