[clang] [clang][layout] Fix unsigned char overflow in ms_struct bitfield layout (PR #181433)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

This seems sensible to me (though it is an ABI breaking change), but at the same time, MSVC doesn't support `_BitInt` yet, so I am slightly concerned there's a chance we implement it this way, Microsoft eventually gets around to `_BitInt` and does something different, and now we've got a second ABI break. CC codegen code owners for more opinions

I don't think we need an ABI version tag for the ABI break in this PR; this seems like a pretty rare circumstance.

The changes should come with a release note in `clang/docs/ReleaseNotes.rst` as a potentially breaking change because it is an ABI change.

https://github.com/llvm/llvm-project/pull/181433