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.

Author: etcwilde

stdlib/public/Concurrency/Task.swift

@@ -834,7 +834,9 @@ internal func _asyncMainDrainQueue() -> Never
 internal func _getMainExecutor() -> Builtin.Executor
 
 @available(SwiftStdlib 5.1, *)
-public func _runAsyncMain(_ asyncFun: @escaping () async throws -> ()) {
+@usableFromInline
+@preconcurrency
+internal func _runAsyncMain(_ asyncFun: @Sendable @escaping () async throws -> ()) {
   Task.detached {
     do {
 #if !os(Windows)