ICU-20072 Move modified = true before return to fix hasMappings()

[dare3path]

This resolves a compiler warning:
collationdatabuilder.cpp:418:20: warning: code will never be executed [-Wunreachable-code].
The modified = true; line after the loop in setPrimaryRangeAndReturnNext() is unreachable due to the return within the for loop and lack of any breaks. Unlike other methods in this file (e.g., maybeSetPrimaryRange()), where modified is set before returning, this leaves modified unset despite trie updates. This could lead to hasMappings() incorrectly returning false when mappings have been added, potentially affecting downstream logic that depends on this flag. Moving modified = true; before the return primary; ensures consistency with the class’s pattern and proper state tracking.

Checklist

• Required: Issue filed: ICU-20072
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• Issue accepted (done by Technical Committee after discussion)
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