Previosly (since commit 9ac98e6e)
it was expected to gradually remove SAL_U/W usage in Windows code
by replacing with reinterpret_cast or changing to some bettertypes.
But as it's useful to make use of fact that LibreOffice and Windows
use compatible representation of strings, this commit puts these
functions to a better-suited o3tl, and recommends that the functions
be consistently used throughout Windows-specific code to reflect the
compatibility and keep the casts safe.

Change-Id: I2f7c65606d0e2d0c01a00f08812bb4ab7659c5f6
Reviewed-on: https://gerrit.libreoffice.org/43150Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>