[Concurrency] Fix race condition in `_runAsyncMain`.
As of the custom main/global executor changes, there is a race in
`_runAsyncMain()` to construct the main executor; if this goes the wrong way,
the IRGen async tests, which use this function, can fail.
Fix by explicitly constructing a task and enqueing it on the main executor,
instead of detaching a task and trying to hop to it.
rdar://148506256
