Allocate dwarf2_cu with new

Allocate dwarf2_cu with new

This changes dwarf2_cu to be allocated with new, and fixes up the
users.

2018-01-17  Tom Tromey  <tom@tromey.com>
	    Simon Marchi  <simon.marchi@ericsson.com>

	* dwarf2read.c (struct dwarf2_cu): Add constructor, destructor.
	(dwarf2_per_objfile::free_cached_comp_units)
	(init_tu_and_read_dwo_dies, init_cutu_and_read_dies)
	(init_cutu_and_read_dies_no_follow): Update.
	(dwarf2_cu::dwarf2_cu): Rename from init_one_comp_unit.
	(dwarf2_cu::~dwarf2_cu): New.
	(free_heap_comp_unit, free_stack_comp_unit): Remove.
	(age_cached_comp_units, free_one_cached_comp_unit): Update.