Provide a definition for OperationPosition::kDown

This isn't necessary in C++17, but C++14 still requires it.

GitOrigin-RevId: 8521850f20dbd9f2926999da84d8a10b52bb2574
diff --git a/lib/Conversion/PDLToPDLInterp/Predicate.cpp b/lib/Conversion/PDLToPDLInterp/Predicate.cpp
index 8983ecb..8d560eb 100644
--- a/lib/Conversion/PDLToPDLInterp/Predicate.cpp
+++ b/lib/Conversion/PDLToPDLInterp/Predicate.cpp
@@ -44,3 +44,8 @@
 OperandGroupPosition::OperandGroupPosition(const KeyTy &key) : Base(key) {
   parent = std::get<0>(key);
 }
+
+//===----------------------------------------------------------------------===//
+// OperationPosition
+
+constexpr unsigned OperationPosition::kDown;