Previously application of plugins was part of fir2ir conversion and
ir actualization was the responsibility of each specific IR backend
facade. Now (after moving IR extensions after IR actualization) those
actions are extracted into separate facade for two purposes:
1. Avoid code duplication in backend facades
2. Step with this facade goes exactly after fir2ir conversion before
irHandlersStep, which allows IR handlers to observe IR which was
actualized and modified by extensions, which is quite useful
^KT-56173
Dmitriy Novozhilov
e7a10057f2