Make more headers self-contained.

A lot of this comes from the new complete type requirement of DenseMap.

llvm-svn: 258956
diff --git a/llvm/tools/llvm-diff/DiffConsumer.h b/llvm/tools/llvm-diff/DiffConsumer.h
index 855f688..36d4a12 100644
--- a/llvm/tools/llvm-diff/DiffConsumer.h
+++ b/llvm/tools/llvm-diff/DiffConsumer.h
@@ -18,6 +18,7 @@
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringRef.h"
+#include "llvm/IR/Value.h"
 #include "llvm/Support/Casting.h"
 #include "llvm/Support/raw_ostream.h"