[AutoDiff upstream] Add differentiability_witness_function instruction. (#29765)

The `differentiability_witness_function` instruction looks up a
differentiability witness function (JVP, VJP, or transpose) for a referenced
function via SIL differentiability witnesses.

Add round-trip parsing/serialization and IRGen tests.

Notes:
- Differentiability witnesses for linear functions require more support.
  `differentiability_witness_function [transpose]` instructions do not yet
  have IRGen.
- Nothing currently generates `differentiability_witness_function` instructions.
  The differentiation transform does, but it hasn't been upstreamed yet.

Resolves TF-1141.

lib/AST/AutoDiff.cpp

@@ -2,7 +2,7 @@
 //
 // This source file is part of the Swift.org open source project
 //
-// Copyright (c) 2019 Apple Inc. and the Swift project authors
+// Copyright (c) 2019 - 2020 Apple Inc. and the Swift project authors
 // Licensed under Apache License v2.0 with Runtime Library Exception
 //
 // See https://swift.org/LICENSE.txt for license information
@@ -12,11 +12,34 @@
 
 #include "swift/AST/AutoDiff.h"
 #include "swift/AST/ASTContext.h"
+#include "swift/AST/Module.h"
 #include "swift/AST/TypeCheckRequests.h"
 #include "swift/AST/Types.h"
 
 using namespace swift;
 
+DifferentiabilityWitnessFunctionKind::DifferentiabilityWitnessFunctionKind(
+    StringRef string) {
+  Optional<innerty> result = llvm::StringSwitch<Optional<innerty>>(string)
+                                 .Case("jvp", JVP)
+                                 .Case("vjp", VJP)
+                                 .Case("transpose", Transpose);
+  assert(result && "Invalid string");
+  rawValue = *result;
+}
+
+Optional<AutoDiffDerivativeFunctionKind>
+DifferentiabilityWitnessFunctionKind::getAsDerivativeFunctionKind() const {
+  switch (rawValue) {
+  case JVP:
+    return {AutoDiffDerivativeFunctionKind::JVP};
+  case VJP:
+    return {AutoDiffDerivativeFunctionKind::VJP};
+  case Transpose:
+    return None;
+  }
+}
+
 void AutoDiffConfig::print(llvm::raw_ostream &s) const {
   s << "(parameters=";
   parameterIndices->print(s);