Promote warnings to errors in array_count_values()

[Girgias]

Split from #4566

[nikic]

Not sure this change is appropriate: Note that this warning is really just a warning, not a proper error condition. The function is still going to count other values, and skip over those with unsupported types. The same is not possible if an exception is thrown.

[SjonHortensius]

I agree, this might not be as straight-forward as it seems. The warning+result allows users to choose between failing and continuing. I only use codebases that convert all warnings to exceptions anyway but this change removes the option that other might choose (using the result even though a warning occured as illustrated by the testcase)