Deprecate runAsyncAndBlock API

We don't want people using this as a means of jumping from synchronous
code to async code. It blocks the thread that it's running on and spawns
a new thread to run the async code.

This has a couple of drawbacks. First, if it blocks a thread that is
needed by the async code (i.e, calling something on the main actor while
blocking the main thread), it is an immediate deadlock. This is a bad
day for everyone. Second, it can easily lead to thread-explosions.

The proper entry-point for async code is to use something of the
following form:

```
@main
struct Main {
  static func main() async {
    // ...
  }
}
```

This should take care of most new code. There are still places where
folks will need to jump from synchronous code to async code. That still
needs to be designed and implemented, but we don't want people to come
to rely on `runAsyncAndBlock`.

Author: etcwilde

stdlib/public/Concurrency/Task.swift

@@ -323,6 +323,7 @@ func _enqueueJobGlobal(_ task: Builtin.Job)
 @_silgen_name("swift_task_isCancelled")
 func isTaskCancelled(_ task: Builtin.NativeObject) -> Bool
 
+@available(*, deprecated)
 @_silgen_name("swift_task_runAndBlockThread")
 public func runAsyncAndBlock(_ asyncFun: @escaping () async -> ())