All of gcc, clang, and Visual Studio provide options to warn when a switch case implicitly falls through to the next case. For gcc and clang it's `-Wimplicit-fallthrough[=level]`, and for Visual Studio it's C26819.

We currently don't enable these warnings at all for Visual Studio. The level 3 warning is enabled for gcc and clang, by `-Wextra`. But for HotSpot we disable this warning. And there are currently 85 such warnings when building HotSpot.

Once we have C++17 `[[fallthrough]]` attributes, those places should be examined and apply the new attribute, or fix any bugs that might be found by that examination. Once that's been done, stop disabling the warning.