Surrounding s4tf specific code with appropiate comments.

Author: vguerra

include/swift/Basic/LangOptions.h

@@ -331,8 +331,10 @@ namespace swift {
     /// `@differentiable` declaration attribute, etc.
     bool EnableExperimentalDifferentiableProgramming = false;
 
+    // SWIFT_ENABLE_TENSORFLOW
     /// Whether to enable forward mode differentiation
     bool EnableExperimentalForwardModeDifferentiation = false;
+    // SWIFT_ENABLE_TENSORFLOW END
 
     /// Whether to enable #quote, #unquote and @quoted.
     bool EnableExperimentalQuasiquotes = false;

include/swift/Option/Options.td

@@ -428,9 +428,12 @@ def disable_bridging_pch : Flag<["-"], "disable-bridging-pch">,
 def enable_experimental_differentiable_programming : Flag<["-"], "enable-experimental-differentiable-programming">,
   Flags<[FrontendOption]>,
   HelpText<"Enable experimental differentiable programming features">;
+// SWIFT_ENABLE_TENSORFLOW
+// NOTE: This flag will be removed when JVP/differential generation is robust.
 def enable_experimental_forward_mode_differentiation : Flag<["-"], "enable-experimental-forward-mode-differentiation">,
   Flags<[FrontendOption]>,
   HelpText<"Enable experimental forward mode differentiation">;
+// SWIFT_ENABLE_TENSORFLOW END
 
 def enable_experimental_quasiquotes : Flag<["-"], "enable-experimental-quasiquotes">,
   Flags<[FrontendOption]>,

lib/Driver/ToolChains.cpp

@@ -231,8 +231,10 @@ static void addCommonFrontendArgs(const ToolChain &TC, const OutputInfo &OI,
                        options::OPT_enable_experimental_dependencies);
   inputArgs.AddLastArg(arguments,
                        options::OPT_experimental_dependency_include_intrafile);
+  // SWIFT_ENABLE_TENSORFLOW
   inputArgs.AddLastArg(
       arguments, options::OPT_enable_experimental_forward_mode_differentiation);
+  // SWIFT_ENABLE_TENSORFLOW END
   inputArgs.AddLastArg(arguments,
                        options::OPT_enable_experimental_quasiquotes);
   inputArgs.AddLastArg(arguments, options::OPT_package_description_version);