Avoid Attr.h includes, CodeGen edition

This saves around 20 includes of Attr.h. Not much.
diff --git a/clang/lib/CodeGen/CGDeclCXX.cpp b/clang/lib/CodeGen/CGDeclCXX.cpp
index b19a728..3baa0a0 100644
--- a/clang/lib/CodeGen/CGDeclCXX.cpp
+++ b/clang/lib/CodeGen/CGDeclCXX.cpp
@@ -10,11 +10,12 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "CodeGenFunction.h"
 #include "CGCXXABI.h"
 #include "CGObjCRuntime.h"
 #include "CGOpenMPRuntime.h"
+#include "CodeGenFunction.h"
 #include "TargetInfo.h"
+#include "clang/AST/Attr.h"
 #include "clang/Basic/CodeGenOptions.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/IR/Intrinsics.h"