Allow direct reflection of a dependent qualified-id having a splicer

[katzdm]

P2996 currently parses the following syntax as an expression:

constexpr auto R = ^[:S:]::inner;

whenever S is dependent on a template parameter. In the case where [:S:]::inner names a type, one could instead write:

constexpr auto R = ^typename [:S:]::inner;

but since P2996R2 removed the namespace [:R:] syntax, if [:S:]::inner names a nested namespace, then one instead must write:

using namespace Alias = [:S:];
constexpr auto R = ^Alias::inner;

Since it is known that the form ^[:S:]::inner will be parsed as an expression of type std::meta::info, it should be feasible to represent it as a reflection having kind e.g., RK_dependent_qualified_id, and to rebuild it as a reflection holding either a namespace, type, template, or expression during tree transform. It would be good to demonstrate the feasibility of this transformation with working code.

[katzdm]

Not planning to adopt this for P2996, and therefore will not implement for the time being.