SI-7839 Final val breaks checkinit build

Scalac does not care to initialize the unused field it emits
for a `final val`.  Users of the accessor method, which supplies
the constant definition inline, will find that -Xcheckinit will
throw.

Therefore, we don't use `final` for the `val phaseName`.
Merge pull request #2943 from som-snytt/issue/7839-phase-ass-chkinit

SI-7839 Final val breaks checkinit build