fdo#74747: Make use of cached string formula results.

Just like we do with cached numeric formula results.

Change-Id: Ib8b311b540caeb47d8c2162a456f7490c5882ad5
(cherry picked from commit aa5ad7b8096cd15a55c467b1a23d03849aeb870d)
Reviewed-on: https://gerrit.libreoffice.org/8526
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>