gdb: add all_tracepoints function

gdb: add all_tracepoints function

Same idea as the previous patches, but to replace the ALL_TRACEPOINTS
macro.  Define a new filtered_iterator that only keeps the breakpoints
for which is_tracepoint returns true (just like the macro did).

I would have like to make it so tracepoint_range yields some
`tracepoint *` instead of some `breakpoint *`, that would help simplify
the callers, who wouldn't have to do the cast themselves.  But I didn't
find an obvious way to do it.  It can always be added later.

It turns out there is already an all_tracepoints function, which returns
a vector containing all the breakpoints that are tracepoint.  Remove it,
most users will just work seamlessly with the new function.  The
exception is start_tracing, which iterated multiple times on the vector.
Adapt this one so it iterates multiple times on the returned range.

Since the existing users of all_tracepoints are outside of breakpoint.c,
this requires defining all_tracepoints and a few supporting types in
breakpoint.h.  So, move breakpoint_iterator from breakpoint.c to
breakpoint.h.

gdb/ChangeLog:

	* breakpoint.h (all_tracepoints): Remove.
	(breakpoint_iterator): Move here.
	(struct tracepoint_filter): New.
	(tracepoint_iterator): New.
	(tracepoint_range): New.
	(all_tracepoints): New.
	* breakpoint.c (ALL_TRACEPOINTS): Remove, replace all users with
	all_tracepoints.
	(breakpoint_iterator): Move to header.
	(all_tracepoints): New.
	* tracepoint.c (start_tracing): Adjust.

Change-Id: I76b1bba4215dbec7a03846c568368aeef7f1e05a