.inheritable fix for 1.6 (#21768)

tests/method/tmethod_issues.nim

@@ -10,7 +10,7 @@ type B
 
 
 # bug #1659
-type Animal = ref object {.inheritable.}
+type Animal {.inheritable.} = ref object
 type Dog = ref object of Animal
 
 method say(a: Animal): auto {.base.} = "wat!"

tests/method/tmultim.nim

@@ -16,7 +16,7 @@ do nothing
 
 # tmultim2
 type
-  TThing = object {.inheritable.}
+  TThing {.inheritable.} = object
   TUnit = object of TThing
     x: int
   TParticle = object of TThing
@@ -49,7 +49,7 @@ staticCollide(a, b)
 
 # tmultim6
 type
-  Thing = object {.inheritable.}
+  Thing {.inheritable.} = object
   Unit[T] = object of Thing
     x: T
   Particle = object of Thing
@@ -81,7 +81,7 @@ method somethin(obj: RootObj) {.base.} =
   echo "do nothing"
 
 type
-  TNode* = object {.inheritable.}
+  TNode* {.inheritable.} = object
   PNode* = ref TNode
 
   PNodeFoo* = ref object of TNode