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/classScopes/fakeOverrides/common.kt

@@ -0,0 +1,7 @@
+platform open class Base {
+    fun base()
+}
+
+platform class Derived : Base {
+    fun derived()
+}

compiler/testData/multiplatform/classScopes/fakeOverrides/jvm.kt

@@ -0,0 +1,7 @@
+impl open class Base {
+    impl fun base() {}
+}
+
+impl class Derived : Base() {
+    impl fun derived() {}
+}

compiler/testData/multiplatform/classScopes/fakeOverrides/output.txt

@@ -0,0 +1,7 @@
+-- Common --
+Exit code: OK
+Output:
+
+-- JVM --
+Exit code: OK
+Output: