Instead of a new attribute `@completionHandlerAsync`, allow the use of
the existing `renamed` parameter of `@available` to specify the
asynchronous alternative of a synchronous function.
No errors will be output from invalid names as `@completionHandlerAsync`
had, but if a function is correctly matched then it will be used to
output warnings when using the synchronous function in an asynchronous
context (as before).
Resolves rdar://80612731
