convert some functions which merely create an OUString on the fly
from a char literal to 'constexpr OUString' literals
Change-Id: I617490baf2d976291b324cc991b59cd18f4b242c
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/166392
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>