When creating the CFGBlocks for a switch statement, we now have the "default"

When creating the CFGBlocks for a switch statement, we now have the "default"
branch ALWAYS be the last successor for a switch-terminated block. This allows
clients to distinguish cases like the following:

switch(...)
  case XXX:
    switch(...) {
      case YYY: ...
    }

  case ZZZ: ..
}

In this case, the block with "case ZZZ:" is the default block for the inner
switch statement, but that case is associated with the outer switch statement,
and not the inner one. Clients can test for this behavior by checking if a
successor block is the last one (and thus just assume that this is the "default"
case).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@47088 91177308-0d34-0410-b5e6-96231b3b80d8