This makes -Wunreachable-code work for programs containing SEH (except for __finally, which is still missing for now).
__try is modeled like try (but simpler since it can only have a single __except or __finally), __except is fairly similar to catch (but simpler, since it can't contain declarations). __leave is implemented similarly to break / continue.
Use the existing addTryDispatchBlock infrastructure (which FindUnreachableCode() in ReachableCode.cpp uses via cfg->try_blocks_begin()) to mark things in the __except blocks as reachable.
Re-use TryTerminatedBlock. This means we add EH edges from calls to the __try block, but not from all other statements. While this is incomplete, it matches LLVM's SEH codegen support. Also, in practice, BuildOpts.AddEHEdges is always false in practice from what I can tell, so we never even insert the call EH edges either.