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

[Axel Lundberg via cfe-commits]

Zonotora wrote:

@zygoloid Thanks for your feedback! Fixed all comments except the last one. I totally agree with you about the distinction of UB checks, and that should be part of a different future PR! 

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