Missing attribute-using prefix in attribute representation

[GorNishanov]

Currently, in IPR we have four forms of attributes:

BasicAttribute = Token
ScopedAttribute = Token :: Token
LabeledAttribute = Token : Attribute
CalledAttribute = Token ( BalancedAttributeSeq )

There is one more element related to attributes supported by C++ grammar that is not currently represented by the IPR.

[ [ attribute-using-prefix-opt attribute-list ] ]
where attribute-using-prefix is using attribute-namespace :

Probably we will need something like:

   struct NamespacedAttribute: Attribute {
      virtual const Token& Namespace() const = 0;
      virtual const BalancedAttributeSequence& attributes() const = 0;
   };

[GabrielDosReis]

Additionally, the current representation misses expansion of attributes (whatever that means).

Furthermore, to support some (misguided) common dialect extensions, there is a need to embed expressions within attributes, explicitly contrary to the requirement of the C++ standards which stipulate that No name lookup is performed on any of the identifiers contained in an attribute-token.

Note that those are orthogonal to the attribute-like notation used for C++20 contracts. But, for generality, it looks like we might have to make embedding of expression in attributes possible.