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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

Thank you for working on this! Please be sure to also add a release note to `clang/docs/ReleaseNotes.rst` so users know about the new diagnostic.

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