[lldb][NFCI] Remove m_being_created from Breakpoint classes (#79716)

The purpose of m_being_created in these classes was to prevent
broadcasting an event related to these Breakpoints during the creation
of the breakpoint (i.e. in the constructor). In Breakpoint and
Watchpoint, m_being_created had no effect. That is to say, removing it
does not change behavior.
However, BreakpointLocation does still use m_being_created. In the
constructor, SetThreadID is called which does broadcast an event only if
`m_being_created` is false. Instead of having this logic be roundabout,
the constructor instead calls `SetThreadIDInternal`, which actually
changes the thread ID. `SetThreadID` also will call
`SetThreadIDInternal` in addition to broadcasting a changed event.