Avoid Attr.h includes, CodeGen edition

This saves around 20 includes of Attr.h. Not much.
diff --git a/clang/lib/CodeGen/SanitizerMetadata.cpp b/clang/lib/CodeGen/SanitizerMetadata.cpp
index ebc9cd5..24ae6c6 100644
--- a/clang/lib/CodeGen/SanitizerMetadata.cpp
+++ b/clang/lib/CodeGen/SanitizerMetadata.cpp
@@ -11,6 +11,7 @@
 //===----------------------------------------------------------------------===//
 #include "SanitizerMetadata.h"
 #include "CodeGenModule.h"
+#include "clang/AST/Attr.h"
 #include "clang/AST/Type.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/IR/Constants.h"