avoid creating temporary in genObjConstr if possible (#7032)

tests/objects/tobjconstr2.nim

@@ -1,3 +1,8 @@
+discard """
+  output: '''42
+Foo'''
+"""
+
 type TFoo{.exportc.} = object
  x:int
 
@@ -48,3 +53,5 @@ type NamedGraphic = object of Graphic2
 
 var ngr = NamedGraphic(kind: Koo, radius: 6.9, name: "Foo")
 echo ngr.name
+
+GC_fullCollect()