[clang] [Clang] Implement the 'counted_by' attribute (PR #76348)

[Kees Cook via cfe-commits]

kees wrote:

Accessing `mi->ints` is unambiguous (it would use the declared `count_ints`) but I'm fine to wait and fix that later. The flex array union is gloriously rare in the kernel. As for whole object, I say pick smallest from all available under bdos(x, 1) and largest for bdos(x, 0), or just say 0. Again, corner case of a corner case.

https://github.com/llvm/llvm-project/pull/76348