MSVC: Improve warning options set

MSVC: Improve warning options set

The previous code had a set of warnings to disable on MSVC.  But some
of these weren't actually enabled by default anyway, only in higher
MSVC warning levels (/W, maps to meson warning_level).  I rearranged
this so that it is clearer in what MSVC warning level a warning would
have been enabled.  Furthermore, sort them numerically within the
levels.

Moreover, we can add a few warning types to the default set, to get a
similar set of warnings that we get by default with gcc or clang (the
equivalents of -Wswitch and -Wformat).

Reviewed-by: Bryan Green <dbryan.green@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/bf060644-47ff-441b-97cf-c685d0827757@eisentraut.org