[clang-tidy] Fix bugprone-misplaced-widening-cast false positive on bitfield assignments (#197554)
When CheckImplicitCasts=true, the checker used the declared type of a
bitfield insread of the actual bitfield width to determine if widening
occurs. This caused false positives when assigning to a bitfield whose
declared type is wider than the source, but whose bitfield width
actually matches the source type. This behavior is fixed.
This PR fixes https://github.com/llvm/llvm-project/issues/197261 issue.
Co-authored-by: Vladislav Aranov <vladislav.aranov@ericsson.com>