Don't lint if the let is already a let-else

We cannot apply the lint for 3-branches like in the added example.

clippy_lints/src/manual_let_else.rs

@@ -73,6 +73,7 @@ impl<'tcx> LateLintPass<'tcx> for ManualLetElse {
             if !in_external_macro(cx.sess(), stmt.span);
             if let StmtKind::Local(local) = stmt.kind;
             if let Some(init) = local.init;
+            if local.els.is_none();
             if init.span.ctxt() == stmt.span.ctxt();
             if let Some(if_let_or_match) = IfLetOrMatch::parse(cx, init);
             then {

tests/ui/manual_let_else.rs

@@ -194,4 +194,7 @@ fn not_fire() {
         };
     }
     create_binding_if_some_nf!(v, g());
+
+    // Already a let-else
+    let Some(a) = (if let Some(b) = Some(Some(())) { b } else { return }) else { panic!() };
 }