[lldb/Breakpoint] Rename StoppointLocation to StoppointSite and drop its relationship...

[lldb/Breakpoint] Rename StoppointLocation to StoppointSite and drop its relationship with BreakpointLocation

Both of BreakpointLocation and BreakpointSite were inherited from StoppointLocation. However, the only thing
they shared was hit counting logic. The patch encapsulates those logic into StoppointHitCounter, renames
StoppointLocation to StoppointSite, and stops BreakpointLocation's inheriting from it.

Differential Revision: https://reviews.llvm.org/D84527