Add a missing forward declaration and namespace qualifier to this

header. This showed up in client code that has a different include
ordering after Chris's shuffle of raw_ostream. All of the Clang code
ends up with a header included before this one that provides the needed
type and using declaration.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@135765 91177308-0d34-0410-b5e6-96231b3b80d8

include/clang/AST/PrettyPrinter.h

@@ -16,6 +16,10 @@
 
 #include "clang/Basic/LangOptions.h"
 
+namespace llvm {
+  class raw_ostream;
+}
+
 namespace clang {
 
 class Stmt;
@@ -25,7 +29,7 @@ class LangOptions;
 class PrinterHelper {
 public:
   virtual ~PrinterHelper();
-  virtual bool handledStmt(Stmt* E, raw_ostream& OS) = 0;
+  virtual bool handledStmt(Stmt* E, llvm::raw_ostream& OS) = 0;
 };
 
 /// \brief Describes how types, statements, expressions, and