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

[Dan Klishch via cfe-commits]

DanShaders wrote:

Just FYI, there's a similar diagnostic `-Wpadded-bitfield` that warns about any padding inserted into bitfields in Itanium mode. They are somehow emitted in the completely different parts of the layout code, so it doesn't show up in the diff at all.

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