FIR: initialize containingClassAttr in light tree builder to match RawFirBuilder

compiler/fir/raw-fir/light-tree2fir/src/org/jetbrains/kotlin/fir/lightTree/converter/DeclarationsConverter.kt

@@ -668,6 +668,8 @@ class DeclarationsConverter(
                     }
                 }
             }
+        }.also {
+            it.containingClassAttr = currentDispatchReceiverType()!!.lookupTag
         }
     }