Revert "Revert "[clang][UBSan] Add implicit conversion check for bitfields"" (#87529)

Reverts llvm/llvm-project#87518

Revert is not needed as the regression was fixed with
1189e879.

I assumed the crash and warning are different issues, but according to
https://lab.llvm.org/buildbot/#/builders/240/builds/26629
fixing warning resolves the crash.