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

[Yeoul Na via cfe-commits]

rapidsna wrote:

Thanks @erichkeane! I moved the allocation into `CountAttributedType::Create`. Does that match what you had in mind?

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