[new AA] implement aaLiteral

This functions will be used to constructing AA values: literals and nulls. When this PR will be merged, I will create PR for dmd, which uses this functions, and when dmd PR will be merged, I'll create PR, which rewrite this functions to use new AA implementation.
Semantically it would make more sense for aaLiteral to return an empty AA literal.
If for some reasons you need a function that returns a null AA, call it `aaNull` or so.