WIP SI-7459 Handle pattern binders used as prefixes in TypeTrees.

Match translation was incorrect for:

  case t => new t.C
  case D(t) => new d.C

We would end up with Types in TypeTrees referring to the wrong symbols, e.g:

  // t7459a.scala
  ((x0$1: this.LM) => {
    case <synthetic> val x1: this.LM = x0$1;
    case4(){
     matchEnd3(new tttt.Node[Any]())
    };
    matchEnd3(x: Any){
      x
  }

Or:
   // t7459b.scala
    ((x0$1: CC) => {
      case <synthetic> val x1: CC = x0$1;
      case4(){
        if (x1.ne(null))
          matchEnd3(new tttt.Node[Any]())
        else
          case5()
      };

This commit:

  - Changes `bindSubPats` to traverse types, as well as terms,
    in search of references to bound symbols
  - Changes `Substitution` to reuse `Tree#substituteSymbols` rather
    than the home-brew substitution from `Tree`s to `Tree`s, if the
    `to` trees are all `Ident`s

I had to dance around the awkward handling of "swatches" (exception
handlers that can be implemented with JVM native type switches) by
duplicating trees to avoid seeing the results of `substituteSymbols`
in `caseDefs` after we abandon that approach if we detect the
patterns are too complex late in the game.