[C++] Support dictionaries directly in coalesce kernel

[asfimport]

This was omitted from ARROW-13390 but still needs doing.

Reporter: David Li / @lidavidm
Assignee: David Li / @lidavidm

Related issues:

• [C++] Reconcile type promotion rules between if_else, case_when, coalesce, select (relates to)
• [C++] Improve type support for 'coalesce' kernel (relates to)
• [C++] Optimize dictionary support in kernels/Support nulls in DictionaryUnifier (is related to)
• [R] Include unused factor levels in coalesce() and if_else() output (is depended upon by)

PRs and other links:

• GitHub Pull Request #11272

_{Note: This issue was originally created as ARROW-14167. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
Issue resolved by pull request 11272
#11272