breakpoint command add currently doesn't support adding commands to more than one breakpoint
at a time.  Enforce this for now (we should relax the requirement when we have a little more time.)

<rdar://problem/13314462>


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@176291 91177308-0d34-0410-b5e6-96231b3b80d8