...which (in LIBO_INTERNAL_ONLY) for Clang expands to [[clang::fallthrough]] in
preparation of enabling -Wimplicit-fallthrough.  (This is only relevant for
C++11, as neither C nor old C++ has a way to annotate intended fallthroughs.)

Could use BOOST_FALLTHROUGH instead of introducing our own SAL_FALLTHROUGH, but
that would require adding back in dependencies on boost_headers to many
libraries where we carefully removed any remaining Boost dependencies only
recently.  (At least make SAL_FALLTHROUGH strictly LIBO_INTERNAL_ONLY, so its
future evolution will not have any impact on the stable URE interface.)  C++17
will have a proper [[fallthroug]], eventually removing the need for a macro
altogether.

Change-Id: I342a7610a107db7d7a344ea9cbddfd9714d7e9ca