[AST] Mark @completionHandlerAsync user-inaccessible

[hamishknight]

This attribute is only really useful to the compiler, so don't expose it to the user through e.g generated interfaces.

Resolves rdar://76685011

[hamishknight]

@swift-ci please test

[hamishknight]

Actually, user-inaccessible isn't what we want here as it'll also block things like code completion...

[bnbarham]

@DougGregor this isn't the correct fix - we should instead mark the ObjC attribute as implicit. This change will:
a) Stop @completionHandlerAsync from appearing in code completion
b) Not show @completionHandlerAsync added in Swift code

I believe @hamishknight was most of the way finished with that change.

Can we revert?

[DougGregor]

Oh, I jumped the gun. We can revert as part of making it implicit, sure, but I'd rather be in the now-merged state than the state prior to the merge. User-inaccessible is more "hidden" than "not usable", and only really hides the attribute from printing and code completion.

[hamishknight]

Yeah sorry for the confusion – I was meaning to update this PR but ran out of time in the day. Opened a PR for the proper fix: #37170