Refactor "do not check impl" flag for multi-platform projects

Instead of LanguageFeature, make it an AnalysisFlag, which is more clear

compiler/util/src/org/jetbrains/kotlin/config/AnalysisFlags.kt

@@ -39,4 +39,7 @@ private operator fun AnalysisFlag.Companion.provideDelegate(instance: Any?, prop
 object AnalysisFlags {
     @JvmStatic
     val skipMetadataVersionCheck by AnalysisFlag
+
+    @JvmStatic
+    val multiPlatformDoNotCheckImpl by AnalysisFlag
 }

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

@@ -52,8 +52,7 @@ enum class LanguageFeature(
 
     Coroutines(KOTLIN_1_1, ApiVersion.KOTLIN_1_1, "https://kotlinlang.org/docs/diagnostics/experimental-coroutines", State.ENABLED_WITH_WARNING),
 
-    MultiPlatformProjects(null, defaultState = State.DISABLED),
-    MultiPlatformDoNotCheckImpl(null, defaultState = State.DISABLED),
+    MultiPlatformProjects(sinceVersion = null, defaultState = State.DISABLED),
 
     ;