[C++] CreateDir should fail if the target exists and is not a directory

[asfimport]

As discussed in #10540 (comment) .

Reporter: Antoine Pitrou / @pitrou
Assignee: Antoine Pitrou / @pitrou

PRs and other links:

• GitHub Pull Request #10541

_{Note: This issue was originally created as ARROW-13092. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
Issue resolved by pull request 10541
#10541