Make a few functions work with base_breakpoint instead of breakpoint

Make a few functions work with base_breakpoint instead of breakpoint

This makes tracepoints inherit from base_breakpoint, since their
locations are code locations.  If we do that, then we can eliminate
tracepoint::re_set and tracepoint::decode_location, as they are doing
the same as the base_breakpoint implementations.

With this, all breakpoint types created by new_breakpoint_from_type
are code breakpoints, i.e., base_breakpoint subclasses, and thus we
can make it return a base_breakpoint pointer.

Finally, init_breakpoint_sal can take a base_breakpoint pointer as
"self" pointer too.  This will let us convert this function to a
base_breakpoint ctor in a following patch.

Change-Id: I3a4073ff1a4c865f525588095c18dc42b744cb54