AST: correct name decoration for swift async functions on Windows

The name decoration scheme on Windows does not have a vendor namespace,
and the decoration scheme is not shared ownership - it is controlled by
Microsoft.  `T` is a reserved identifier for an unknown calling
convention.  The `W` identifier has been discussed with Microsoft
offline and is reserved as `Swift_3` as the identifier for the swift
async calling convention.  Adjust the name decoration accordingly.