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

[Andrew Kaster via cfe-commits]

ADKaster wrote:

https://github.com/llvm/llvm-project/pull/70978, which split -Wpadded into -Wpadded and -Wpadded-bitfield, may be relevant here

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