[AutoDiff upstream] [SIL] Add differentiable function instructions.

Add `differentiable_function` and `differentiable_function_extract`
instructions.

`differentiable_function` creates a `@differentiable` function-typed
value from an original function operand and derivative function operands
(optional).

`differentiable_function_extract` extracts either the original or
derivative function value from a `@differentiable` function.

The differentiation transform canonicalizes `differentiable_function`
instructions, filling in derivative function operands if missing.

Resolves TF-1139 and TF-1140.

include/swift/SIL/SILBuilder.h

@@ -2164,6 +2164,23 @@ public:
   // Differentiable programming instructions
   //===--------------------------------------------------------------------===//
 
+  DifferentiableFunctionInst *createDifferentiableFunction(
+      SILLocation Loc, IndexSubset *ParameterIndices, SILValue OriginalFunction,
+      Optional<std::pair<SILValue, SILValue>> JVPAndVJPFunctions = None) {
+    return insert(DifferentiableFunctionInst::create(
+        getModule(), getSILDebugLocation(Loc), ParameterIndices,
+        OriginalFunction, JVPAndVJPFunctions, hasOwnership()));
+  }
+
+  /// Note: explicit extractee type may be specified only in lowered SIL.
+  DifferentiableFunctionExtractInst *createDifferentiableFunctionExtract(
+      SILLocation Loc, NormalDifferentiableFunctionTypeComponent Extractee,
+      SILValue Function, Optional<SILType> ExtracteeType = None) {
+    return insert(new (getModule()) DifferentiableFunctionExtractInst(
+        getModule(), getSILDebugLocation(Loc), Extractee, Function,
+        ExtracteeType));
+  }
+
   /// Note: explicit function type may be specified only in lowered SIL.
   DifferentiabilityWitnessFunctionInst *createDifferentiabilityWitnessFunction(
       SILLocation Loc, DifferentiabilityWitnessFunctionKind WitnessKind,