[PATCH] [analyzer] alpha.different.MissingBreak checker

[Jordan Rose]

We already have this as a warning in Clang: -Wimplicit-fallthrough. The problem is that we have no way to silence it in any mode except C++11 (where we have a `[[clang::fallthrough]]` attribute), so it currently doesn't do anything in other language modes. There have been some past discussions on cfe-dev about turning it on anyway, possibly with idioms to silence the warning (like a comment containing the word "fallthrough").

http://reviews.llvm.org/D3912