Modularize: Include some required headers

DenseMaps require the definition of a type to be available when using a
pointer to that type as a key to know how many bits are available for
tombstone/etc.

llvm-svn: 317360
diff --git a/llvm/lib/Bitcode/Writer/ValueEnumerator.h b/llvm/lib/Bitcode/Writer/ValueEnumerator.h
index 7301870..011356c 100644
--- a/llvm/lib/Bitcode/Writer/ValueEnumerator.h
+++ b/llvm/lib/Bitcode/Writer/ValueEnumerator.h
@@ -18,6 +18,8 @@
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/UniqueVector.h"
 #include "llvm/IR/Attributes.h"
+#include "llvm/IR/Metadata.h"
+#include "llvm/IR/Type.h"
 #include "llvm/IR/UseListOrder.h"
 #include <cassert>
 #include <cstdint>