[-Wunreachable-code] always treat 'case:' and 'default:' cases as reachable.

This is a heuristic.  Many switch statements, although they look covered
over an enum, may actually handle at runtime more values than in the enum.

This is overly conservative, as there are some cases that clearly
can be ruled as being clearly unreachable, e.g. 'switch (42) { case 1: ... }'.
We can refine this later.

llvm-svn: 202436