Fix #2012: Fix not reachable definitions of default methods

[WojciechMazur]

This PR resolves #2012. Default method implementations were up until now not used in the reachability check process.

This issue was also observed #2022, #2023

[WojciechMazur]

I've probably run of iteration on a version other than 2.11.x which led me to the wrong conclusions. The bug still exists.
I'm converting this to draft as I'm going to fix it anyway it the next days.

[WojciechMazur]

There is a single error still existing, but it disappears when java.util.Iterator.remove() is marked as JavaDefaultMethod.

can't call D6removeuEO on @"T82org.scalanative.testsuite.javalib.lang.IterableDefaultTest$$anon$3$$anon$6$$anon$7"

The iterator in this test does not override the remove() method what results in the mentioned error. Personally, I would say that is expected behavior.

I propose to merge this PR, followed by merging #2004 when it's ready.

[LeeTibbert]

Wojciech,

LGTM!

Thank you for doing this work. It is a lot smaller, tighter, and elegant than the NirGenStat.scala work that I about
to submit this week. I learned a lot of compiler internals, so the time was well spent.

I tested (test-all, which includes scripted) with my zoo of files having JavaDefaultMethods (Map.scala,
Iterator.scala, Comparator.scala, Predicate.scala, Function.scala, Iterable.scala) with both Scala 2.11.12
and Scala 2.13.4. All passed. Looking good....

If it is agreeable to you, I will wait for this PR to be merged, and then rebase PR 2004 and, upon passing,
promote it out of WIP.

Once that PR builds, I will start rolling though the rest of my JavaDefaultMethod related PRs in the queue,
rebasing, rebuilding, & promoting.

Sound like a plan? Please advise if there is a better way to dance.

[WojciechMazur]

@LeeTibbert let's do this in a way you described