Make collect_probes return an std::vector

Make collect_probes return an std::vector

Change collect_probes so it returns an std::vector<bound_probe> instead
of a VEC(bound_probe_s).  This allows removing some cleanups.  It also
seems like enable_probes_command and disable_probes_command were not
freeing that vector.

The comparison function compare_probes needs to be updated to return a
bool indicating whether the first parameter is "less than" the second
parameter.

I defined two constructors to bound_probe.  The default constructor is
needed, for example, so the instance in struct bp_location can be
constructed without parameters.  The constructor with parameters is
useful so we can use emplace_back and pass the values directly.

The s390 builder on the buildbot shows a weird failure that I can't
explain:

../../binutils-gdb/gdb/elfread.c: In function void probe_key_free(bfd*, void*):
../../binutils-gdb/gdb/elfread.c:1346:8: error: types may not be defined in a for-range-declaration [-Werror]
   for (struct probe *probe : *probes)
        ^~~~~~

I guess it's a bug with that specific version< of the compiler, since no
other gcc gives me that error.  It is using:

  g++ (GCC) 6.3.1 20161221 (Red Hat 6.3.1-1)

Any idea about this problem?

gdb/ChangeLog:

	* probe.h (struct bound_probe): Define constructors.
	* probe.c (bound_probe_s): Remove typedef.
	(DEF_VEC_O (bound_probe_s)): Remove VEC.
	(collect_probes): Change return type to std::vector, remove
	cleanup.
	(compare_probes): Return bool, change parameter type.  Change
	semantic to "less than".
	(gen_ui_out_table_header_info): Change parameter to std::vector
	and update.
	(exists_probe_with_pops): Likewise.
	(info_probes_for_ops): Update to std::vector change.
	(enable_probes_command): Likewise.
	(disable_probes_command): Likewise.