Avoid Attr.h includes, CodeGen edition

This saves around 20 includes of Attr.h. Not much.
diff --git a/clang/lib/CodeGen/MicrosoftCXXABI.cpp b/clang/lib/CodeGen/MicrosoftCXXABI.cpp
index 800d02d..aff4613 100644
--- a/clang/lib/CodeGen/MicrosoftCXXABI.cpp
+++ b/clang/lib/CodeGen/MicrosoftCXXABI.cpp
@@ -19,6 +19,7 @@
 #include "CodeGenModule.h"
 #include "CodeGenTypes.h"
 #include "TargetInfo.h"
+#include "clang/AST/Attr.h"
 #include "clang/AST/CXXInheritance.h"
 #include "clang/AST/Decl.h"
 #include "clang/AST/DeclCXX.h"