Avoid various uses of Option<Span> in favor of using DUMMY_SP in the few cases that used None

compiler/rustc_mir_transform/src/known_panics_lint.rs

@@ -261,7 +261,7 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
         // manually normalized.
         let val = self.tcx.try_normalize_erasing_regions(self.param_env, c.const_).ok()?;
 
-        self.use_ecx(|this| this.ecx.eval_mir_constant(&val, Some(c.span), None))?
+        self.use_ecx(|this| this.ecx.eval_mir_constant(&val, c.span, None))?
             .as_mplace_or_imm()
             .right()
     }