[clang] Add an off-by-default warning to complain about MSVC bitfield padding (PR #117428)

[Oliver Hunt via cfe-commits]

ojhunt wrote:

Oh, my goal for this warning is to have us turn it on in the llvm+clang builds, so we can then move away from endless use of unsigned bitfields and just use enums as nature intended :D (cc @AaronBallman )

https://github.com/llvm/llvm-project/pull/117428