Case modification fails for Unicode characters

[Dennis Williamson]

s=łódź; echo "${s^^} ${s~~}"'
łóDź ŁÓDŹ

The to-upper and the undocumented toggle operators should produce
identical output in this situation, but only the toggle works
correctly.

This is in en_US.UTF-8, but also reported in pl_PL.utf-8. In Bash
4.2.24 and Bash 4.0.33.

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