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

[Oliver Hunt via cfe-commits]

================

----------------
ojhunt wrote:

No one commented on this test case, but for the record:

* It runs in both ms_bitfields and sane (:D) mode
* The static assertions are used to force struct layout
* We assert the structs are the same size or different sizes depending on layout mode, so we can then just ensure the structs with different layouts get warnings.

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