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

[Yeoul Na via cfe-commits]

rapidsna wrote:

> Maybe reference the discussion from #212906 in the description.
> 
> I tried to check if this requires any serialization changes, since the storage of the CoupledDecls is now different, but it seems this implementation detail doesn't affect the de/serialization.
> 
> LGTM. Hopefully one of the maintainers can do a deeper review as well.

@zmodem Thanks for the quick review! I updated the PR description to reference relevant discussions!

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