SI-5353 unsound Array lubs.

Attempt 1 was:

  mergePrefixAndArgs tried too hard to finesse the Array lub. When
  there is more than one distinct Array element type being lubbed,
  abandon the attempt to erase to any common Array type, because the
  storage must match all the element types and Nothing will always
  break this. Simply erase to AnyRef.

Attempt 2 was:

  The previous fix, while possibly still appropriate, did not
  suffice to fight off Nothing-based array lubs. Took eugene's
  advaice and went after them in erasure.

I can maybe remove Attempt 1.