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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

Please add test coverage for compound assignment and increment/decrement.

This seems like a reasonable extension of the existing integer truncation checks, but we might want to consider giving it a unique name.  Otherwise, people using integer truncation checks will have trouble upgrading their compiler.

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