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
This does several different things to improve how platforms are described in availability diagnostics:
• Mentions the platform in diagnostics for platform-specific @available(unavailable) attributes.
• Replaces “OS X” with “macOS”.
• Replaces “fooOS application extension” with “application extensions for fooOS”.
• Replaces “on fooOS” with “in fooOS”.
Fixes <rdar://problem/49963341>.
