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

[Eli Friedman via cfe-commits]

https://github.com/efriedma-quic commented:

I'm a bit concerned about the lack of C++ testcases, since you're making changes to some C++-only codepaths.

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