Add locking around the m_owners collection in the breakpoint site.  If we are in the middle of "BreakpointLocation::ShouldStop" we don't
want other commands (like "break disable") to mutate the owners of this breakpoint out from under us.

<rdar://problem/17255589>


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@211136 91177308-0d34-0410-b5e6-96231b3b80d8