[clang] [Clang] Generate the GEP instead of adding AST nodes (PR #73730)

[Bill Wendling via cfe-commits]

bwendling wrote:

Okay, this was tougher than I thought. I make sure that we're not causing any new side-effects in the code. I try as much as possible to use previously generated code when accessing the `counted_by` variable. I also corrected how offsets were calculated.

There are some a few issues having to do with the `union_of_fams` in the testcase. But the rest of this code should be ready to go.

https://github.com/llvm/llvm-project/pull/73730