There is lots of (Windows-only) code that relied on sal_Unicode being the same
as wchar_t, and the best change may be different in each case (and doing the
changes may be somewhat error prone).  So for now add SAL_U/SAL_W scaffolding
functions to sal/types.h, remove their uses one by one again, and finally drop
those functions again.

Change-Id: I2cc791bd941d089901abb5f6fc2f05fbc49e65ea
Reviewed-on: https://gerrit.libreoffice.org/36077Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>