JS backend: basic support for class literals.

Added:
* the ability to get KClass using class literals (`::class`);
* the ability to get KClass from JsClass and vice versa;
* the ability to get simpleName.

 #KT-13345 Fixed

js/js.translator/testData/box/reflection/kClassToAndFromJsClass.kt

@@ -0,0 +1,29 @@
+package foo
+
+import kotlin.reflect.KClass
+
+fun check(k: KClass<*>, j: JsClass<*>) {
+    assertNotEquals(null, k)
+    assertNotEquals(null, j)
+
+    assertEquals(k.js, j)
+    assertEquals(k, j.kotlin)
+    assertEquals(j, j.kotlin.js)
+    assertEquals(k, k.js.kotlin)
+}
+
+fun jsClassbyName(name: String) = js("JS_TESTS").foo[name]
+
+fun box(): String {
+    check(A::class, jsClassbyName("A"))
+    check(B::class, jsClassbyName("B"))
+    check(O::class, jsClassbyName("O").constructor)
+    check(I::class, jsClassbyName("I"))
+    check(E::class, jsClassbyName("E"))
+    check(E.X::class, jsClassbyName("E").X.constructor)
+    check(E.Y::class, jsClassbyName("E").Y.constructor)
+// TODO uncomment after KT-13338 is fixed
+//    check(E.Z::class, jsClassbyName("E").Z.constructor)
+
+    return "OK"
+}