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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

Precommit CI seems to have found some related failures that should be investigated. I'd love to hear opinions from the codegen code owners on this, as well as @zygoloid as UBSan code owner. I'm not opposed to the changes, though I do find it somewhat strange to add them to UBSan given that they're not undefined behavior (even though there's precedent).

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