message: Use precomputed TyLayout from `machine.layouts`
  And add comment documenting successful return value from
  `mach_timebase_info`.
diff: @@ -173,13 +173,12 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
  // `mach_absolute_time`, we don't need to scale the absolute
  // time.
  let (numer, denom) = (1,1);
  - let uint32_layout = this.layout_of(this.tcx.types.u32)?;
  let imms = [
  - immty_from_int_checked(numer, uint32_layout)?,
  - immty_from_int_checked(denom, uint32_layout)?
  + immty_from_int_checked(numer, this.machine.layouts.u32)?,
  + immty_from_int_checked(denom, this.machine.layouts.u32)?
  ];
  this.write_packed_immediates(info, &imms)?;
  - Ok(0)
  + Ok(0) // KERN_SUCCESS
  }
  }
  