[clang] [WIP][Clang] Add __builtin_get_counted_by builtin (PR #102549)

[Bill Wendling via cfe-commits]

bwendling wrote:

There's still some cleanup to do. Firstly, the code to get the `MemberExpr` for the flexible array member. It's sloppy.

https://github.com/llvm/llvm-project/pull/102549