Put back raw_ostream.h include.

Some configurations of MSVC get horribly confused by the forward declaration
and explode.

llvm-svn: 208493

clang/include/clang/AST/Decl.h

@@ -25,6 +25,7 @@
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/Optional.h"
 #include "llvm/Support/Compiler.h"
+#include "llvm/Support/raw_ostream.h"
 
 namespace clang {
 struct ASTTemplateArgumentListInfo;