We were getting an assert because somebody was making a watchpoint that was
neither read nor write.  Tighten up the checking so this isn't possible.

<rdar://problem/14111167>


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@184245 91177308-0d34-0410-b5e6-96231b3b80d8