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
When calling a witness table accessor, IRGen was forcing the
conforming type to have complete metadata, even though only abstract
metadata is required for that query. This could cause cyclic metadata
dependencies when checking conditional conformances.
Fixes SR-5958.
