Enables `-checked-async-objc-bridging` flag by default starting
from swift 6 language mode to provide enhanced debugging and
runtime checking for the correct usage of continuations passed
in a call to ObjC from Swift.
Because `CheckedContinuation` is not a @frozen struct we have
to use `Any` to store it in @block_storage indirectly. If the
flag is enabled, we'd emit a block storage with `Any` and
initialize the existential with stack allocated `CheckedContinuation`
formed from `UnsafeContinuation`. Inside of the completion handler
`Any` is going to be projected and cast back to `CheckedContinuation`.
