Include variant in AdjointValue.h

d971f12 added a usage of std::variant and std::holds_alternative to
SILOptimizer/Differentiation/AdjointValue.h, but did not include variant
directly. This caused issues while building on Linux.

Author: etcwilde

include/swift/SILOptimizer/Differentiation/AdjointValue.h

@@ -25,6 +25,8 @@
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/Support/Debug.h"
 
+#include <variant>
+
 namespace swift {
 namespace autodiff {