Implement --file-names-only (#8788)

* flatc builds and seems to work, some of the extra targets are having linker errors

* fix build system

* pipeline failures

* un-rename files

* refactor to use unique_ptr

* typo

* rm make_unique, add comments

* fix cmake

---------

Co-authored-by: Wouter van Oortmerssen <aardappel@gmail.com>

src/idl_gen_swift.cpp

@@ -191,7 +191,8 @@ class SwiftGenerator : public BaseGenerator {
 
     const auto filename = GeneratedFileName(path_, file_name_, parser_.opts);
     const auto final_code = code_.ToString();
-    return SaveFile(filename.c_str(), final_code, false);
+    return parser_.opts.file_saver->SaveFile(filename.c_str(), final_code,
+                                             false);
   }
 
   void GenerateCode() {