Use assert_crate_local for a more explicit error

`assert_crate_local` does the same as the previous `if let` but with a
more explicit error message if it's not a `ClearCrossCrate::Set`.

clippy_lints/src/redundant_clone.rs

@@ -209,11 +209,7 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for RedundantClone {
             if !used_later {
                 let span = terminator.source_info.span;
                 let scope = terminator.source_info.scope;
-                let node = if let mir::ClearCrossCrate::Set(scope_local_data) = &mir.source_scopes[scope].local_data {
-                    scope_local_data.lint_root
-                } else {
-                    unreachable!()
-                };
+                let node = mir.source_scopes[scope].local_data.as_ref().assert_crate_local().lint_root;
 
                 if_chain! {
                     if let Some(snip) = snippet_opt(cx, span);