Before this commit, questionable optimization existed which
unwrapped string interpolating call with single argument to this argument.
However, this led to source element loss and the necessity of sub-hacks.
In this commit we dropped this optimization (anyway user can remove
this single-expression string template in code if needed) to keep
source elements intact.
- To discriminate what's already been generated, use the set of declaration
instead of names (it's obviously more correct)
- Make it possible to set more then one overridden (base)
Mikhail Glukhikh