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/_common.kt

@@ -20,3 +20,18 @@ enum class E {
 
 @JsName("Q")
 class R
+
+fun check(x: Any, y: Any, shouldBeEqual: Boolean = true, shouldBeSame: Boolean = true) {
+    assertNotEquals(null, x)
+    assertNotEquals(null, y)
+    if (shouldBeEqual) {
+        assertEquals(x, y)
+
+        if (shouldBeSame && x !== y) {
+            fail("Expected same instances, got expected = '$x', actual = '$y'")
+        }
+    }
+    else {
+        assertNotEquals(x, y)
+    }
+}