Enable 2021 compatibility lints for all in-tree code

This just applies the suggested fixes from the compatibility warnings,
leaving any that are in practice spurious in. This is primarily intended to
provide a starting point to identify possible fixes to the migrations (e.g., by
avoiding spurious warnings).

A secondary commit cleans these up where they are false positives (as is true in
many of the cases).

Author: Mark-Simulacrum

clippy_utils/src/lib.rs

@@ -990,7 +990,10 @@ pub fn can_move_expr_to_closure(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) ->
         captures: HirIdMap::default(),
     };
     v.visit_expr(expr);
-    v.allow_closure.then(|| v.captures)
+    v.allow_closure.then(|| {
+        let _ = &v;
+        v.captures
+    })
 }
 
 /// Returns the method names and argument list of nested method call expressions that make up