Support "-Xno-check-impl" argument, check only real declarations

Use "-Xno-check-impl" to suppress checking whether the platform declaration
implementation has the "impl" modifier.

Do not check presence of fake overrides from platform class in the impl class,
otherwise there would be a lot of errors about the fact that
equals/hashCode/toString are not marked with the "impl" modifier

compiler/testData/multiplatform/simple/js.kt

@@ -1,5 +1,5 @@
 impl class Printer {
-    fun print(message: String) {
+    impl fun print(message: String) {
         println("JS says: " + message)
     }
 }