The VS2017 started to succeed while executing
test/IRGen/async/throwing.swift, but it is marked as XFAIL for
OS=windows-msvc. To avoid failing the test in VS2019, only mark it as
unsupported in MSVC_VER=15.0 (VS2017), which will avoid running the
test.
`async_task_detached.swift` seems to fail with an `ACCESS_VIOLATION`
exit status, but seems to pass in VS2019, so also mark it as
`UNSUPPORTED` for VS2017 only.
All these problems are tracked in SR-14333.
Previously, the error stored in the async context was of type SwiftError
*. In order to enable the context to be callee released, make it
indirect and change its type to SwiftError **.
rdar://71378532
