SI-7433 Fix spurious warning about catching control throwable

In the same vein as SI-6994, we have to be careful not to warn
about synthetic code. In that case, the spurious warnings came
because we warned in the typechecker, which was also called in
erasure. In this case, we are issuing the warning in Uncurry,
so we must be mindful of the pattern matchers translations of
non-trivial catch patterns, which look like:

    case (ex8 @ _) => {
    <synthetic> val x5: Throwable = ex8;
    case11(){
      if ({
        case14(){
          if (x5.$isInstanceOf[NullPointerException]())
            matchEnd13(true)
          else
            case15()
        };
        case15(){
          if (x5.$isInstanceOf[RuntimeException]())
            matchEnd13(true)
          else
            case16()
        };
        case16(){
          matchEnd13(false)
        };
        matchEnd13(x: Boolean){
          x
        }
      })

This commit detects that `ex8` is synthetic and disables the warning.
Merge pull request #2653 from retronym/ticket/7433

SI-7433 Fix spurious warning about catching control throwable