Codegen: use type forward declarations more aggresively. Fixes #7339 (#7340)

Do not emit object definition it if used only by ref or ptr

tests/ccgbugs/mymodule.nim

@@ -0,0 +1,12 @@
+type
+  MyRefObject* = ref object
+    s: string
+
+
+proc newMyRefObject*(s: string): MyRefObject =
+  new(result)
+  result.s = s
+  
+proc `$`*(o: MyRefObject): string =
+  o.s
+  

tests/ccgbugs/tforward_decl_only.nim

@@ -0,0 +1,15 @@
+discard """
+ccodecheck: "\\i !@('struct tyObject_MyRefObject'[0-z]+' {')"
+output: "hello"
+"""
+
+# issue #7339 
+# Test that MyRefObject is only forward declared as it used only by reference
+
+import mymodule
+type AnotherType = object
+  f: MyRefObject 
+
+let x = AnotherType(f: newMyRefObject("hello"))
+echo $x.f
+