[C++] Support concatenation of UnionArrays

[asfimport]

#3746 adds support for concatenation of arrays, but UnionArrays are not supported.

Reporter: Ben Kietzman / @bkietz
Assignee: Matthijs Brobbel / @mbrobbel

Related issues:

• [C++] add support for UnionArrays to Take and Filter (blocks)

PRs and other links:

• GitHub Pull Request #11843

_{Note: This issue was originally created as ARROW-4975. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
This should be almost trivial for sparse unions and would require a bit more care for dense unions.

[asfimport]

Ben Kietzman / @bkietz:
Issue resolved by pull request 11843
#11843