.inheritable fix for 1.6 (#21768)

tests/types/tfinalobj.nim

@@ -3,7 +3,7 @@ discard """
 """
 
 type
-  TA = object {.pure, final.}
+  TA {.pure, final.} = object
     x: string
 
 var

tests/types/tinheritref.nim

@@ -11,11 +11,11 @@ ob = T[int](elem: 23)
 doAssert ob.elem == 23
 
 type
-  TTreeIteratorA* = ref object {.inheritable.}
+  TTreeIteratorA* {.inheritable.} = ref object
 
   TKeysIteratorA* = ref object of TTreeIteratorA  #compiles
 
-  TTreeIterator* [T,D] = ref object {.inheritable.}
+  TTreeIterator* [T,D] {.inheritable.} = ref object
 
   TKeysIterator* [T,D] = ref object of TTreeIterator[T,D]  #this not