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
Mark imported `@completionHandlerAsync` attrs as
implicit, which avoids printing them in generated
interfaces. And for the sake of completion,
serialize the implicit bit in case it's used
elsewhere in the future.
To make sure we continue to print
`@completionHandlerAsync` attributes explicitly
written by the user in Swift, add a SourceKit
interface test.
Resolves rdar://76685011
