[clang] [clang] Implement CWG2627 Bit-fields and narrowing conversions (PR #78112)

[Mital Ashok via cfe-commits]

MitalAshok wrote:

@AaronBallman I think I've addressed all your comments, thanks for the review! Could you please merge this for me if there is nothing else?

https://github.com/llvm/llvm-project/pull/78112