Add PRIVATE_TO_THIS visibility to KtSymbolWithVisibility

This visibility is used for some cases where it isn't allowed to access
to private members of the object from other instances of the same object
because of the type parameter variance.

For almost all clients this visibility is the same as PRIVATE

idea/idea-frontend-api/src/org/jetbrains/kotlin/idea/frontend/api/symbols/markers/KtSymbolWithVisibility.kt

@@ -12,8 +12,12 @@ interface KtSymbolWithVisibility {
 sealed class KtSymbolVisibility {
     object PUBLIC : KtSymbolVisibility()
     object PRIVATE : KtSymbolVisibility()
+    object PRIVATE_TO_THIS : KtSymbolVisibility()
     object PROTECTED : KtSymbolVisibility()
     object INTERNAL : KtSymbolVisibility()
     object UNKNOWN : KtSymbolVisibility()
     object LOCAL : KtSymbolVisibility()
-}
+}
+
+fun KtSymbolVisibility.isPrivateOrPrivateToThis(): Boolean =
+    this == KtSymbolVisibility.PRIVATE || this == KtSymbolVisibility.PRIVATE_TO_THIS