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/fir/providers/src/org/jetbrains/kotlin/fir/types/ConeTypeContext.kt

@@ -113,9 +113,9 @@ interface ConeTypeContext : TypeSystemContext, TypeSystemOptimizationContext, Ty
         return this as? ConeDynamicType
     }
 
-    override fun FlexibleTypeMarker.asRawType(): RawTypeMarker? {
-        require(this is ConeFlexibleType)
-        return this as? ConeRawType
+    override fun KotlinTypeMarker.isRawType(): Boolean {
+        require(this is ConeKotlinType)
+        return this.isRaw()
     }
 
     override fun FlexibleTypeMarker.upperBound(): SimpleTypeMarker {