[clang] Use range-based for to iterate over fields in record layout, NFC (PR #122029)

[Boaz Brickner via cfe-commits]

bricknerb wrote:

Thanks for doing this!
FWIW, I think splitting this to smaller PRs would be easier. For example:
1. `getFieldIndex()` inlining.
2. Passing fields instead of their indexes and iterate over fields.
3. Use `std::next()` instead of defining a `Next` variable.
4. Rename `isNoUniqueAddress` to `isNonVirtualBaseType`.

https://github.com/llvm/llvm-project/pull/122029