Use std::vector for traceframe_info::tvars

Straightforward change to get rid of a VEC.  We need to new/delete
traceframe_info instead of malloc/free it.  I found three places that
allocate a traceframe_info (ctf_traceframe_info, tfile_traceframe_info
and parse_traceframe_info) and only one that frees it
(free_traceframe_info).

gdb/ChangeLog:

	* tracepoint.h (struct traceframe_info) <tvars>: Change type to
	std::vector<int>.
	* tracepoint.c (free_traceframe_info): Deallocate with delete.
	(traceframe_info_start_tvar): Adjust to vector change.
	(parse_traceframe_info): Allocate with new.
	* ctf.c (ctf_traceframe_info): Allocate with new, adjust to
	vector change.
	* tracefile-tfile.c (build_traceframe_info): Adjust to vector
	change.
	tfile_traceframe_info): Allocate with new.
	* mi/mi-main.c (mi_cmd_trace_frame_collected): Adjust to vector
	change.