Fix 13493: compute union of child types for mirror

[bishabosha]

based on #15006

for each part of the union compute a separate child type, then compute the lub.

closes #13493

[bishabosha]

@odersky looking at this implementation it makes me wonder if we want to support union types at all, what do you think?

[dwijnand]

I'm feel like I'm convinved, but would be good to get Martin to check as well.

[bishabosha]

for clarity, the reason that each part of the union is computed with separate constraints is to avoid making an intersection type

[odersky]

I am not so familiar with mirrors anymore to be able to say whether or not we should support mirrors for unions.

[bishabosha]

I think it would make it easier to completely ban them - especially with all the type computations necessary to produce mirrors.

[bishabosha]

closing in favour of #15279