Adds implementation flag for swift (#7202)

* Adds implementation flag for swift

Forces internal flag when using @_implementationOnly in swift

Fixes access type for verifier functions & encoder functions

Updates generated code

* Addresses PR comments & adds a code gen dir within the swift tests

* Adds test case for no-include

* Fixes code gen script

Removes prefix

src/flatc.cpp

@@ -86,6 +86,8 @@ const static FlatCOption options[] = {
   { "", "scoped-enums", "",
     "Use C++11 style scoped and strongly typed enums. Also implies "
     "--no-prefix." },
+  { "", "swift-implementation-only", "",
+    "Adds a @_implementationOnly to swift imports" },
   { "", "gen-inclues", "",
     "(deprecated), this is the default behavior. If the original behavior is "
     "required (no include statements) use --no-includes." },
@@ -490,6 +492,8 @@ int FlatCompiler::Compile(int argc, const char **argv) {
         opts.java_checkerframework = true;
       } else if (arg == "--gen-generated") {
         opts.gen_generated = true;
+      } else if (arg == "--swift-implementation-only") {
+        opts.swift_implementation_only = true;
       } else if (arg == "--gen-json-emit") {
         opts.gen_json_coders = true;
       } else if (arg == "--object-prefix") {