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

[Vitaly Buka via cfe-commits]

vitalybuka wrote:

> Is is UB?

I guess it follows existing convention `implicit-integer-truncation` is convenience check and `implicit-signed-integer-truncation` is UB

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