E.g. ScModelObj::GetFormatter() can be rather deep in the call chain
and it just doesn't make sense to pass ScInterpreterContext* to
all the relevant places (and it's a question if it makes sense
to pass it around at all, googling shows that thread_local is not
really _that_ slow).

Change-Id: I9114ef8a10d82a10968391718099edccde7a2663
Reviewed-on: https://gerrit.libreoffice.org/63184
Tested-by: Jenkins
Reviewed-by: Luboš Luňák <l.lunak@collabora.com>