Fix scala.meta.tests.semanticdb.OccurrenceSuite -> example/Synthetic.scala

[tgodzik]

It seems that yield now returns an additional local symbol for Scala 2.13.15, which is not correct.

  for {
    a <- scala.concurrent.Future.successful(1)
    b <- scala.concurrent.Future.successful(2)
    if a < b
  } yield a

last a is found to be a separate symbol (which might be true internally)