TypeSystemContext: Rework raw types processing

All the current usages are about checking if the type is raw,
also in K2 it is simply incorrect to assume that only flexible types
might be raw

core/descriptors/src/org/jetbrains/kotlin/types/checker/ClassicTypeSystemContext.kt

@@ -153,9 +153,9 @@ interface ClassicTypeSystemContext : TypeSystemInferenceExtensionContext, TypeSy
         return this as? DynamicType
     }
 
-    override fun FlexibleTypeMarker.asRawType(): RawTypeMarker? {
-        require(this is FlexibleType, this::errorMessage)
-        return this as? RawType
+    override fun KotlinTypeMarker.isRawType(): Boolean {
+        require(this is KotlinType, this::errorMessage)
+        return this is RawType
     }
 
     override fun FlexibleTypeMarker.upperBound(): SimpleTypeMarker {