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/util/src/org/jetbrains/kotlin/config/LanguageVersionSettings.kt

@@ -36,6 +36,7 @@ enum class LanguageFeature(val sinceVersion: LanguageVersion?) {
 
     // Experimental features
     MultiPlatformProjects(null),
+    MultiPlatformDoNotCheckImpl(null),
     ;
 
     val presentableText: String