[clang][Interp] Add IntegralAP for arbitrary-precision integers (#65844)

This adds `IntegralAP` backing the two new primtypes `IntAP` (unsigned
arbitrary-precision int) and `IntAPS` (same but signed).

We use this for `int128` support (which isn't available on all host
systems we support AFAIK) and I think we can also use this for `_BitInt`
later.