Replace Task.h with right (5.6) version
I seem to have copied over the wrong version of Task.h. There is an ABI
mismatch in the size of AsyncContext due to the removal of Flags. This
resulted in programs crashing when running against the backdeploy
library and should have crashed when running on the swift 5.6 runtime.
The successResultPointer pointer was set in AsyncTask::waitFuture, but
with the wrong layout. When the pointer was read in the concurrency
backdeploy library, it was at a different offset, and thus contained a
nullptr.
I pulled the AsyncContextKind and AsyncContextFlags from the old
MetadataValues.h into Task.h as they were removed in commit
aca744b21165a20655502b563a6fa54c2c83efdf, but are necessary with the
flags included.
