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

[Oliver Hunt via cfe-commits]

ojhunt wrote:

> So I take it we decided not to enable it by default in `-fms-compatibility` mode then?

I don't believe it is appropriate to do so. The intent of this warning is to indicate MSVC compatibility issues when building in non-ms-compatibility modes. The existing padding warnings already trigger for these layouts in the relevant ms compatibility modes

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