Safe Transmute: Fix propagation of errors

- Make sure that the most specific Reason is the one that bubbles up when we
  are folding over the `Answer` tree. `Reason::DstIsBitIncompatible` is the
  least specific, so that should be used only when there isn't anything else
  available.
- Small fixes where we used the wrong Reason variant.
- Tiny cleanups

compiler/rustc_trait_selection/src/traits/select/confirmation.rs

@@ -329,7 +329,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                         )
                     };
 
-                    // // FIXME(bryangarza): Check src.mutability or dst.mutability to know whether dst -> src obligation is needed
+                    // FIXME(bryangarza): Check src.mutability or dst.mutability to know whether dst -> src obligation is needed
                     Ok(vec![make_obl(src.ty, dst.ty), make_obl(dst.ty, src.ty)])
                 }
             }