Handle C++11 attribute namespaces automatically.

Now, as long as the 'Namespaces' variable is correct inside Attr.td, the
generated code will correctly admit a C++11 attribute only when it has the
appropriate namespace(s).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@158661 91177308-0d34-0410-b5e6-96231b3b80d8