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

[Axel Lundberg via cfe-commits]

Zonotora 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).

@AaronBallman CI Linux is successful, but "Trigger build on clang-ci" seems to be failing on unrelated format and windows build? I also find it strange that these checks are included in UBSan. However, it might be better to refactor this in a future PR, as you say, there are reasons to include it in UBSan now because of the current `-fsanitize=implicit-conversion` flag. How do we move this forward and merge the PR?

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