SI-7716 Exclude patmat synthetics from bounds checking

Consider this pattern match translation, that occurs *before* refchecks:

    scala> val e: java.lang.Enum[_] = java.util.concurrent.TimeUnit.SECONDS

    scala> e match { case x => x }
    <console>:9: error: type arguments [_$1] do not conform to class Enum's type parameter bounds [E <: Enum[E]]
                  e match { case x => x }
                  ^
    [[syntax trees at end of                 refchecks]] // <console>
    package $line5 {
              case <synthetic> val x1: Enum[_$1] = $line3.$read.$iw.$iw.e;
              case4(){
                matchEnd3(x1)
              };
              matchEnd3(x: Enum[_$1]){
                x
              }

RefChecks turns a blind eye to the non-conformant type `Enum[_$1]` in
the label defs because of 65340ed4ad2. (Incidentally, that is far
too broad, as I've noted in SI-7756.)

This commit extends this exception to cover the synthetic ValDef `x1`.

Commit log watchers might notice the similarities to SI-7694.

Review by @adriaanm / @JamesIry. I think this one makes sense for 2.10.3. It was a regression from 2.9.x.