[LLDB][NativePDB] Use pointer and not pointee size for pointer constants (#180987)

Follow-up from
https://github.com/llvm/llvm-project/pull/180612#pullrequestreview-3780142672

This was introduced in 2af3416618e6 and not fundamentally changed since.
The review (https://reviews.llvm.org/D54452) doesn't mention any
motivation for why we would want the pointee size/signedness here. As
@Nerixyz points out, only a null pointer can be `S_CONSTANT` anyway.

GitOrigin-RevId: 5fef36767fe8d852a65ef3a4c020c677bc80799b