Add locking around the m_owners collection in the breakpoint site. If we are...

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>

llvm-svn: 211136