Remove needless Attr.h include from DeclCXX.h, NFC

This actually has no impact on the build, because TypeLoc.h includes
Attr.h. However, DeclCXX.h has no need of it, so go ahead and remove it.
The final step in this patch series is to split Attr.h and Attrs.h.

Author: rnk

clang/include/clang/AST/DeclCXX.h

@@ -17,7 +17,6 @@
 
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/ASTUnresolvedSet.h"
-#include "clang/AST/Attr.h"
 #include "clang/AST/Decl.h"
 #include "clang/AST/DeclBase.h"
 #include "clang/AST/DeclarationName.h"