mangle: do not mangle reserved class (#2317)

fixes #2316

test/compress/harmony.js

@@ -804,3 +804,39 @@ object_spread_of_sequence: {
         console.log({ ...o || o });
     }
 }
+
+// issue 2316
+class_name_can_be_preserved_with_reserved: {
+    mangle = {
+        reserved: ['Foo']
+    }
+    input: {
+        function x() {
+            class Foo {};
+            Foo.bar;
+            class Bar {};
+            Bar.foo;
+        }
+
+        function y() {
+            var Foo = class Foo {};
+            Foo.bar();
+            var Bar = class Bar {};
+            Bar.bar();
+        }
+    }
+    expect: {
+        function x() {
+            class Foo {}
+            Foo.bar;
+            class a{}
+            a.foo
+        }
+        function y() {
+            var Foo = class Foo {};
+            Foo.bar();
+            var a = class a{};
+            a.bar()
+        }
+    }
+}