Add a missing forward declaration of PassManagerBase.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@159566 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/Passes.h

@@ -24,6 +24,7 @@ namespace llvm {
   class FunctionPass;
   class MachineFunctionPass;
   class PassInfo;
+  class PassManagerBase;
   class TargetLowering;
   class TargetRegisterClass;
   class raw_ostream;