Attribute syntax TODO: support using

[elbeno]

Since C++17, attributes can have using in them:

[[using name::space: attr]]

Also (this is admittedly strange) the C++ grammar doesn't introduce an attribute with a single [[ token but with two tokens [ [.
(Yes, this means it's really annoying to call an immediately-invoked lambda inside a call to operator[]).

[Tsche]

The given example is invalid, attribute-namespaces must be identifiers (dcl.attr.grammar), therefore scope resolution operators may not appear in an attribute-namespace.

Anyway, #278 implements this.