[C++][CI] StructToStructSubset test failure with libc++ 22.1.1

[pitrou]

Describe the bug, including details regarding any error messages, version, and platform.

This has started appearing on MinGW CLang CI, see https://github.com/apache/arrow/actions/runs/23467758420/job/68341855187 for example

The root cause is in a libc++ change in std::multimap, see llvm/llvm-project#152370 (comment) . It's unclear yet whether our code or the libc++ change is incorrect.

Component(s)

C++, Continuous Integration

[pitrou]

@lidavidm @WillAyd FYI

[pitrou]

In any case, this can probably be worked around by calling lower_bound for the given key and then extract on the returned iterator.