These tests are marked XFAIL or UNSUPPORTED because either the tests:
require libc annotation, require Mach-O support, don't recognize calls to
swift-autolink-extract, requires porting alongside Linux, or rely on simd
which is not present.
Additionally, explicit REQUIRES for tsan/asan/fuzzer are added to some
tests, since OpenBSD does not support these sanitizers or fuzzers, since
it's nicer to mark that with REQUIRES rather than XFAIL.
1) Enable tests that use `import Dispatch` on Linux. Add substitution
`%import-libdispatch` that needs to be used for all cross-platform
tests (i.e., tests that are intended to be run on other platforms
than Darwin) that do `import Dispatch` or enable thread sanitizer.
2) Make sure as many existing Dispatch and TSan tests as possible run on
Linux. Mark tests that would require substantial work with
`UNSUPPORTED: OS=linux-gnu`.
3) Add integration-style Swift test that shows that TSan finds a simple
race when using `Dispatch.async` incorrectly. A more complete test
suite for TSan's libdispatch support lives on the LLVM/compiler-rt
side.
rdar://problem/49177535
