[clang] [clang] Make LazyOffsetPtr more portable (PR #112927)

[Sam James via cfe-commits]

thesamesam wrote:

I appreciate it's not exactly a straightforward change given the discussion it provoked, but any chance you'd be okay with a backport to 19? Clang isn't really usable on ppc32 without this. We could pull it in just for ppc32 downstream but we generally dislike doing conditional patching. WDYT?

https://github.com/llvm/llvm-project/pull/112927