[llvm] Add -Wms-bitfield-padding warning when possible (PR #139828)

[Oliver Hunt via llvm-commits]

ojhunt wrote:

@mstorsjo @Endilll @IanWood1 given you're doing stage2+ builds would you folk mind seeing if this warning triggers for you? after applying https://github.com/llvm/llvm-project/pull/139825 if possible

https://github.com/llvm/llvm-project/pull/139828