[clang] [clang] Ensure pointers passed to runtime support functions are correctly signed (PR #98276)

[Anton Korobeynikov via cfe-commits]

asl wrote:

> I notice the r+, but I haven't fully addressed this feedback - are you suggesting corrections in a follow up patch, or just conceptually approving with this PR? :D

I think this patch is fine to go as-is. We can re-consider that `MaybeSigned` abstraction further on if it could appear in multiple places.

https://github.com/llvm/llvm-project/pull/98276