An emplace_hint with the iterator pointing at end() doesn't really help,
so rather attempt an insert with a temporary value.

Also check if the upper-case string we got back is the same as the input
string, in which case, we can save memory by mapping the input string to
itself.

Change-Id: I40b9e2b65a831e44c4b88d51d835242a47d8a86d
Reviewed-on: https://gerrit.libreoffice.org/72516
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>