The `call` argument to `diagnoseDeclAsyncAvailability()` may be `nullptr` when the structure of the AST prevents `getEnclosingApplyExpr()` from finding the enclosing apply. The diagnostics implementation introduced with https://github.com/apple/swift/pull/60663 needs to tollerate this, similar to the rest of `diagnoseDeclAsyncAvailability()`.
Resolves rdar://100862513
Implicitly add the @completionHandlerAsync attribute for ObjCMethodDecl
that have a completion handler. Adds a link from the non-async to the
async function for use in diagnostics and refactorings.
Resolves rdar://74665226
