This includes a revert of commit 9808486a "Avoid
-Werror,-Wimplicit-fallthrough with clang-cl", and also undoes older MSVC
workarounds (that would start to cause "warning C4702: unreachable code" now).

Change-Id: If72bf336e12e0a2db589857e8875003dae861977
Reviewed-on: https://gerrit.libreoffice.org/67248
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>