Cleaning up _runAsyncMain a bit
Cleaning up _runAsyncMain a bit

I'm making two cleanups here. First, the closure going into
`_runAsyncMain` needs to be `@Sendable` or passing it to the task is not
safe. This will also result in a warning being emitted.
Second, I'm making this @usableFromInline and `internal`. This function
is around for legacy reasons, but it's part of the ABI, so we can't pull
it out entirely, but we don't want folks using it.

JGiola/swift,benlangmuir/swift,glessard/swift,benlangmuir/swift,rudkx/swift,ahoppen/swift,gregomni/swift,atrick/swift,ahoppen/swift,glessard/swift,roambotics/swift,atrick/swift,atrick/swift,JGiola/swift,roambotics/swift,benlangmuir/swift,JGiola/swift,glessard/swift,apple/swift,roambotics/swift,rudkx/swift,apple/swift,apple/swift,gregomni/swift,ahoppen/swift,glessard/swift,rudkx/swift,rudkx/swift,rudkx/swift,benlangmuir/swift,rudkx/swift,roambotics/swift,JGiola/swift,ahoppen/swift,atrick/swift,apple/swift,glessard/swift,ahoppen/swift,glessard/swift,gregomni/swift,roambotics/swift,gregomni/swift,apple/swift,gregomni/swift,apple/swift,ahoppen/swift,benlangmuir/swift,JGiola/swift,atrick/swift,benlangmuir/swift,JGiola/swift,atrick/swift,roambotics/swift,gregomni/swift