[lldb][NFCI] Change return type of BreakpointIDList::GetBreakpointIDAtIndex (#77166)

There are 2 motivations here:
1.) There is no need to hand out constant references to BreakpointIDs,
they are only 8 bytes big. In addition, every use of this method already
  makes a copy anyway.
2.) Each BreakpointIDList held onto an invalid BreakpointID specifically
to
  prevent lifetime issues. Returning a value means you can return an
  invalid BreakpointID instead of needing to allocate storage for an
  invalid BreakpointID.