SI-5900 Pending test to show that SI-7886 persists

    qbin/scalac test/pending/neg/t7886b.scala && qbin/scala Test
    java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.String
        at Test$$anon$1.accept(t7886b.scala:15)
        at Test$.g(t7886b.scala:9)
SI-5900 Fix pattern inference regression

This commit does not close SI-5900. It only addresses a regression
in 2.11 prereleases caused by SI-7886.

The fix for SI-7886 was incomplete (as shown by the last commit)
and incorrect (as shown by the regression in pos/t5900a.scala and
the fact it ended up inferring type parameters.)

I believe that the key to fixing this problem will be unifying
the inference of case class constructor patterns and extractor
patterns.

I've explored that idea:

  https://gist.github.com/retronym/7704153
  https://github.com/retronym/scala/compare/ticket/5900

But didn't quite get there.
Merge pull request #3523 from adriaanm/rebase-3514

 Fix pattern inference regression