Noise reduction + minor enhance in TreeCheckers.

Misc irrelevant work, which I can only offer as-is. It lowers
the noise in -Ycheck:* output and performs some common sense
chillaxes like not screaming ERROR IN INTERNAL CHECKING! WE'RE
ALL GOING TO DIE! when a tree doesn't hit all nine points at
the Jiffy Tree.

You can see some reasonably well reduced symbol flailing if
you run the included pending tests:

  test/partest --show-diff test/pending/pos/treecheckers

Example output,

Out of scope symbol reference {
      tree  TypeTree             Factory[Traversable]
  position  OffsetPosition       test/pending/pos/treecheckers/c5.scala:3
  with sym  ClassSymbol          Factory: Factory[CC]
   and tpe  ClassArgsTypeRef     Factory[Traversable]
   encl(1)  ModuleSymbol         object Test5
    ref to  AbstractTypeSymbol   X (<deferred> <param>)
}

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