Add hook for phase when a TreeTransform is run.
Add hook for phase when a TreeTransform is run.

Achieved by overridanle method treeTransformPhase in TreeTransform.
This is currently by default the current phase. We should migrate it
to the one after current phase.

lampepfl/dotty,svalaskevicius/dotty,dotty-staging/dotty,vsalvis/dotty,sjrd/dotty,densh/dotty,sjrd/dotty,dotty-staging/dotty,auvik/dotty,VladimirNik/dotty,som-snytt/dotty,sjrd/dotty,dotty-staging/dotty,spetz911/dotty,lampepfl/dotty,som-snytt/dotty,angi2/dotty,spetz911/dotty,som-snytt/dotty,dotty-staging/dotty,densh/dotty,reactormonk/dotty,lampepfl/dotty,angi2/dotty,som-snytt/dotty,yusuke2255/dotty,sjrd/dotty,svalaskevicius/dotty,reactormonk/dotty,auvik/dotty,sjrd/dotty,lampepfl/dotty,som-snytt/dotty,dotty-staging/dotty,AlexSikia/dotty,lampepfl/dotty,vsalvis/dotty,folone/dotty,folone/dotty,AlexSikia/dotty,yusuke2255/dotty,VladimirNik/dotty