resetAttrs now always erases This.tpe

The symbol of This, if it points to a package class, isn't touched, just
as usual, so that our Select(Select(Select(...))) => This(...) optimization
works fine with attr reset.

However the tpe is now erased, so that subsequent reflective compilation
doesn't spuriously fail when seeing that some subtrees of a tree being
compiled are typed.

Erasing the tpe doesn't pose even a tiniest problem, because, as it can
be seen in typedThis, type is trivially reconstructed from the symbol.
adds some comments to resetAttrs
Merge pull request #2134 from scalamacros/topic/resetattrs-this

resetAttrs now always erases This.tpe