OUStringLiteral should be declared constexpr, to enforce
that it is initialised at compile-time and not runtime.
This seems to make a different at least on Visual Studio
Change-Id: I1698f5fa22ddb480347c2f4d444530c2e0e88d92
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/153499
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>