Workaround for existential bind / resetAttrs interaction

The original trees for pattern matching Binds are somewhat unusual.

    pat = (s#4999 @ (_: Seq#41[_#5000]))
                    `------- Typed ---'
          `--------- Bind -------------'

    val Typed(_, tpt: TypeTree) = <<(_: Seq#41[_#5000])>>
    val AppliedTypeTree(<<scala.Seq#41>>, (arg: TypeTree) :: Nil) = tpt.original
    val Bind(_#5000, EmptyTree) = arg.original

This commit manually resets the typed `_#5000` to `Ident(tpename.WILDCARD)`.

I'm not sure yet whether scalac ought to present a fully untyped original here.

Review by @phaller