Use std::vector for traceframe_info::memory

Straightforward change from a VEC to std::vector.  This allows making
the destruction of a traceframe_info trivial.

I added a constructor with parameters to mem_range to be able to
emplace_back directly with the values.  It is necessary to leave a
default constructor there because mem_range is still used in a VEC.

gdb/ChangeLog:

	* memrange.h (struct mem_range): Add constructors.
	* tracepoint.h (struct traceframe_info) <memory>: Change type to
	std::vector<mem_range>.
	* tracepoint.c (free_traceframe_info): Don't manually free
	vector.
	(traceframe_info_start_memory): Adjust to vector change.
	(traceframe_available_memory): Likewise.
	* tracefile-tfile.c (build_traceframe_info): Likewise.
	* ctf.c (ctf_traceframe_info): Likewise.