[clang] [compiler-rt] [clang][UBSan] Add implicit conversion check for bitfields (PR #75481)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> > Also, I don't have commit access (first PR here), someone has to commit on my behalf!
> 
> @AaronBallman

Thank you for mentioning this! I'll commit on your behalf, but I'd like to hear from @zygoloid on the ABI question. It would also be nice if @efriedma-quic signed off on the testing changes (I think they're sufficient, but he may have other cases in mind).

https://github.com/llvm/llvm-project/pull/75481