SI-7020 Deterministic warnings for pattern matcher, take 2

The previous swing at determinism, ebb01e05cbe4, made decent contact but
apparently didn't hit it out of the park. The test wavered every hundred or so
runs, as witnessed occasionally in nightly builds or pull request validation.

I setup a test to run neg/7020.scala a few hundred times, and could trigger
the failure reliably.

I then swept through the pattern matcher in search of HashMap and HashSet
creation, and changed them all to the Linked variety. The results of that are
published in retronym#ticket/7020-3 [1].

This commit represents the careful whittling down of that patch to the minimal
change required to exhibit determinism.

[1] https://github.com/retronym/scala/compare/ticket/7020-3