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

[Oliver Hunt via cfe-commits]

https://github.com/ojhunt approved this pull request.

This lgtm, but given @AaronBallman's comments on ABI impact he should probably confirm that the release note is sufficient 

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