Assortiment of cleanups and comments around the backend

[lrytz]

No description provided.

[lrytz]

Tests went fine, review by @retronym

[lrytz]

Thanks @retronym for insisting on a before-after comparison: I found a serious issue and have to give up on introducing ownerAtCurrentPhase. The problem is that the current phase in saveOriginalOwner can be anything, not necessarily the phase in which the owner was/is changed.

For example, the tree transformation in the lambdalift phase, which lifts definitions (and modifies their symbol's owner), is executed afterOwnPhase. Also, an owner might be modified in an info transformer, which are executed lazily.

I'll go back to just save the originalOwner for now. It's really hard to touch/improve incrementally around flags and owners that depend change depending on the phase :( I'm open to discuss things, but I think my time budget for that has run out long ago..

[lrytz]

-Ybackend:GenBCode build ok

[lrytz]

removed ownerAtCurrentPhase again. review by @retronym, @gkossakowski

[gkossakowski]

LGTM.

In the future please emphasize the why over what when writing commit messages. A year from now it's going to be a lot easier to work out what has been changed in the diff than why it was done. I'd like to emphasize this point in our policy: https://github.com/scala/scala/wiki/Pull-Request-Policy