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/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
+