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

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/types/IrTypeSystemContext.kt

@@ -54,7 +54,7 @@ interface IrTypeSystemContext : TypeSystemContext, TypeSystemCommonSuperTypesCon
 
     override fun FlexibleTypeMarker.asDynamicType() = this as? IrDynamicType
 
-    override fun FlexibleTypeMarker.asRawType(): RawTypeMarker? = null
+    override fun KotlinTypeMarker.isRawType(): Boolean = false
 
     override fun FlexibleTypeMarker.upperBound(): SimpleTypeMarker {
         return when (this) {