Update include in ASTContext.cpp.

Author: rxwei

lib/AST/ASTContext.cpp

@@ -17,7 +17,6 @@
 #include "swift/AST/ASTContext.h"
 #include "ForeignRepresentationInfo.h"
 #include "SubstitutionMapStorage.h"
-#include "swift/AST/AutoDiff.h"
 #include "swift/AST/ClangModuleLoader.h"
 #include "swift/AST/ConcreteDeclRef.h"
 #include "swift/AST/DiagnosticEngine.h"
@@ -29,6 +28,7 @@
 #include "swift/AST/GenericSignature.h"
 #include "swift/AST/GenericSignatureBuilder.h"
 #include "swift/AST/ImportCache.h"
+#include "swift/AST/IndexSubset.h"
 #include "swift/AST/KnownProtocols.h"
 #include "swift/AST/LazyResolver.h"
 #include "swift/AST/ModuleLoader.h"