[clang] [BoundsSafety] Allow 'counted_by' attribute on pointers in structs in C (PR #90786)

[Yeoul Na via cfe-commits]

rapidsna wrote:

@bwendling @kees Wait. `ATOM_PPLIB_STATE_V2` is also a struct with flexible array member? This is concerning because `ucNumEntries * sizeof(ATOM_PPLIB_STATE_V2)` is not the correct size anyway. Do you know the semantics of this structure?

https://github.com/llvm/llvm-project/pull/90786