Add a forward defn for Module since it's no longer in Value.h

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@2344 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/GlobalValue.h

@@ -12,6 +12,7 @@
 
 #include "llvm/User.h"
 class PointerType;
+class Module;
 
 class GlobalValue : public User {
   GlobalValue(const GlobalValue &);             // do not implement