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

[Oliver Hunt via cfe-commits]

ojhunt wrote:

> @ojhunt If by commit access you mean the ability to merge this PR myself, then I don't have it yet.

Ah, I just saw you haven't yet added the release notes Aaron requested, so you'll need to do those first

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