Inconsistency when using C++11 `friend` syntax for types

[foonathan]

• cppast version: latest one
• parser: libclang_parser
• clang version: 4.0

C++11 introduced a new syntax to mark types as friends, friend foo instead of friend class foo.
When parsing friend foo it will be treated as if friend class foo was written.
But when parsing friend ns::foo, it will be detected properly.