Avoid Attr.h includes, CodeGen edition

This saves around 20 includes of Attr.h. Not much.
diff --git a/clang/lib/CodeGen/CGBlocks.cpp b/clang/lib/CodeGen/CGBlocks.cpp
index 6a1a739..b0295ce 100644
--- a/clang/lib/CodeGen/CGBlocks.cpp
+++ b/clang/lib/CodeGen/CGBlocks.cpp
@@ -19,6 +19,7 @@
 #include "CodeGenModule.h"
 #include "ConstantEmitter.h"
 #include "TargetInfo.h"
+#include "clang/AST/Attr.h"
 #include "clang/AST/DeclObjC.h"
 #include "clang/CodeGen/ConstantInitBuilder.h"
 #include "llvm/ADT/SmallSet.h"