Quickfix for NOTHING_TO_OVERRIDE: change function signature to match one of super function's signatures.

This implements "Modify method parameters/return type to match one of method in super-classes." solution from https://docs.google.com/document/d/1b9I-7VUMq8As9gd4agU8hPIXrBauplG8_4vLK5Pv-zw/edit#heading=h.6g59oaej69hc, this time without breaking any tests (hopefully).

It's exactly the same as https://github.com/JetBrains/kotlin/pull/240, but rebased onto current master and with another commit which fixes the tests. 

When I was fixing the tests, I noticed that one change may be controversial (see the comment https://github.com/sapal/kotlin/commit/505562b77c973b95b309f3117299e1d67930070d#commitcomment-3014405) - please take look at it.