[AutoDiff] Simplify conditions enabling differentiable programming. (#30765)

Previously, two conditions were necessary to enable differentiable programming:
- Using the `-enable-experimental-differentiable-programming` frontend flag.
- Importing the `_Differentiation` module.

Importing the `_Differentiation` module is the true condition because it
contains the required compiler-known `Differentiable` protocol. The frontend
flag is redundant and cumbersome.

Now, the frontend flag is removed.
Importing `_Differentiation` is the only condition.

lib/Sema/DerivedConformanceAdditiveArithmetic.cpp

@@ -56,9 +56,9 @@ static StringRef getMathOperatorName(MathOperator op) {
 bool DerivedConformance::canDeriveAdditiveArithmetic(NominalTypeDecl *nominal,
                                                      DeclContext *DC) {
   // Experimental `AdditiveArithmetic` derivation must be enabled.
-  auto &ctx = nominal->getASTContext();
-  if (!ctx.LangOpts.EnableExperimentalAdditiveArithmeticDerivedConformances)
-    return false;
+  if (auto *SF = DC->getParentSourceFile())
+    if (!isAdditiveArithmeticConformanceDerivationEnabled(*SF))
+      return false;
   // Nominal type must be a struct. (No stored properties is okay.)
   auto *structDecl = dyn_cast<StructDecl>(nominal);
   if (!structDecl)