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
1189e87951e59a81ee097eae847c06008276fef1.
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.