[mlir] [libc] [llvm] [clang] [lldb] [NFC][ObjectSizeOffset] Use classes instead of std::pair (PR #76882)

[Bill Wendling via llvm-commits]

bwendling wrote:

@jdoerfert I reworked the structs to use inheritance instead of template specialization. It should work better now and not duplicate so much.

https://github.com/llvm/llvm-project/pull/76882