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/cli/js/jsExtraHelp.out

@@ -4,6 +4,7 @@ where advanced options include:
   -Xrepeat <count>           Repeat compilation (for performance analysis)
   -Xplugin <path>            Load plugins from the given classpath
   -Xmulti-platform           Enable experimental language support for multi-platform projects
+  -Xno-check-impl            Do not check presence of 'impl' modifier in multi-platform projects
 
 Advanced options are non-standard and may be changed or removed without any notice.
 OK

compiler/testData/cli/jvm/extraHelp.out

@@ -17,6 +17,7 @@ where advanced options include:
   -Xrepeat <count>           Repeat compilation (for performance analysis)
   -Xplugin <path>            Load plugins from the given classpath
   -Xmulti-platform           Enable experimental language support for multi-platform projects
+  -Xno-check-impl            Do not check presence of 'impl' modifier in multi-platform projects
 
 Advanced options are non-standard and may be changed or removed without any notice.
 OK

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:

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)
     }
 }

compiler/testData/multiplatform/simple/jvm.kt

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