[AutoDiff upstream] Add -enable-experimental-differentiable-programming frontend flag. (#27446)

This flag will enable all experimental differentiable programming features.
The default will be `true` on tensorflow branch but `false` on master branch.

Features will first be updated on tensorflow branch to use this flag, before
being upstreamed to master. The goal is to achieve a minimal code diff between
the two branches.

The [TF-820](https://bugs.swift.org/browse/TF-820) master issue tracks upstreaming differentiable programming.

---

Rationale: we chose to add a frontend flag rather than a `build-script`/CMake flag for easier testing. Differentiable programming `lit` tests can be run by specifying this additional flag without recompiling the compiler and standard library.

[Forum discussion on upstreaming differentiable programming.](https://forums.swift.org/t/upstreaming-differentiable-attribute-and-differentiable-protocol/26821)

Author: dan-zheng

include/swift/Basic/LangOptions.h

@@ -325,6 +325,10 @@ namespace swift {
     /// set to true.
     bool ExperimentalDependenciesIncludeIntrafileOnes = false;
 
+    /// Whether to enable experimental differentiable programming features:
+    /// `@differentiable` declaration attribute, etc.
+    bool EnableExperimentalDifferentiableProgramming = false;
+
     /// Sets the target we are building for and updates platform conditions
     /// to match.
     ///

include/swift/Option/Options.td

@@ -424,6 +424,11 @@ def disable_bridging_pch : Flag<["-"], "disable-bridging-pch">,
   Flags<[HelpHidden]>,
   HelpText<"Disable automatic generation of bridging PCH files">;
 
+// Experimental feature options
+def enable_experimental_differentiable_programming : Flag<["-"], "enable-experimental-differentiable-programming">,
+  Flags<[FrontendOption]>,
+  HelpText<"Enable experimental differentiable programming features">;
+
 // Diagnostic control options
 def suppress_warnings : Flag<["-"], "suppress-warnings">,
   Flags<[FrontendOption]>,