fdo#77990: Intern strings for hybrid strings during import.

Change-Id: I269497cac645e486ac08bb2b011df1b5b23dc021
(cherry picked from commit 1899bd0c3b8a16ede2de0125e762b23ba013d81f)
Reviewed-on: https://gerrit.libreoffice.org/9264
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>