fix jsonutils macro with generic case object (#24429)

split from #24425

The added test did not work previously. The result of `getTypeImpl` is
the uninstantiated AST of the original type symbol, and the macro
attempts to use this type for the result. To fix the issue, the provided
`typedesc` argument is used instead.

lib/std/jsonutils.nim

@@ -112,7 +112,7 @@ macro initCaseObject(T: typedesc, fun: untyped): untyped =
   else: raiseAssert $t.kind # xxx `nnkPtrTy` could be handled too
   doAssert t2.kind == nnkRecList
   result = newTree(nnkObjConstr)
-  result.add sym
+  result.add T
   for ti in t2:
     if ti.kind == nnkRecCase:
       let key = ti[0][0]