Refactor Driver.Session (Ways and Flags)

This is preliminary work for #14335.

I'm working on making interpreter related code independent of DynFlags so that we can use both the internal and an external interpreter at the same time. This series of patch refactor interpreterDynamic and interpreterProfiled so that they take an Interp parameter intead of DynFlags (second commit). To do this I had to make some refactoring (see first commit). While I was at it, I've replaced [Way] with Set Way because we always use it as a set (third commit).