[clang] [Clang][counted_by] Load the count through unions by byte offset (PR #228309)

[Amaan Qureshi via cfe-commits]

amaanq wrote:

I confirm that I've read the policies mentioned in the comment above, especially the LLVM AI Tool Use Policy, and that I've noted my AI usage in the PR description.

https://github.com/llvm/llvm-project/pull/228309