[ORC] Fix compile error (llvm#164411)

This fixes the following error we're getting internally, perhaps only
with the latest clang:

llvm/include/llvm/ExecutionEngine/Orc/WaitingOnGraph.h:136:24: error:
missing '#include <__fwd/vector.h>'; default argument of 'vector' must
be defined before it is used

This is a fix for llvm#164340.

Author: googlewalt

llvm/include/llvm/ExecutionEngine/Orc/WaitingOnGraph.h

@@ -19,6 +19,7 @@
 #include "llvm/Support/raw_ostream.h"
 
 #include <algorithm>
+#include <vector>
 
 namespace llvm::orc::detail {