[clang] [BoundsSafety][NFC] Allow CountAttributedType's count to be filled in later (PR #223267)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

I'm on the fence a touch yet as to whether we should have an `IncompleteCountAttributedType`, like we do with `dependent types`, but I think I'm willing to see how this turns out. 

The incomplete type that is later filled in seems like it would be a much more effective mechanism rather than transforming the containing objects.

So a bit of a 'nit' here, else this is pretty reasonable for me. 

https://github.com/llvm/llvm-project/pull/223267