Bug 1570499 - Part 1: Replace MOZ_FALLTHROUGH macro with C++17's [[fallthrough]] attribute. r=froydnj

This changeset is a simple find and replace of `MOZ_FALLTHROUGH` and `[[fallthrough]]`.

Unfortunately, the MOZ_FALLTHROUGH_ASSERT macro (to assert on case fallthrough in debug builds) is still necessary after switching from [[clang::fallthrough]] to [[fallthrough]] because:

* MOZ_ASSERT(false) followed by [[fallthrough]] triggers a -Wunreachable-code warning in DEBUG builds
* but MOZ_ASSERT(false) without [[fallthrough]] triggers a -Wimplicit-fallthrough warning in NDEBUG builds.

Differential Revision: https://phabricator.services.mozilla.com/D56440

--HG--
extra : moz-landing-system : lando