Add a --crash option to not.

Now the two possible uses of not are
* not cmd
  Will return true if cmd doesn't crash and returns false.
* not --crash cmd
  Will return true if cmd crashes.

It will be used/tested in a followup commit for the clang crash recovery
testing.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185678 91177308-0d34-0410-b5e6-96231b3b80d8