Change how dwarf2_per_cu_data is deleted

Address sanitizer pointed out that the patch to use 'delete' for
dwarf2_per_cu_data introduced a bug -- now it is possible to delete a
signatured_type using a pointer to its base class.

This patch fixes the problem by introducing a deleter and a unique_ptr
specialization.  A virtual destructor would be more ordinary here, but
it seemed wasteful to add a vtable just for this purpose.  If virtual
methods are ever needed here, we can revisit this.

2021-05-17  Tom Tromey  <tromey@adacore.com>

	* dwarf2/read.h (struct dwarf2_per_cu_data_deleter: New.
	(dwarf2_per_cu_data_up): New typedef.
	(struct dwarf2_per_bfd) <allocate_per_cu>: Change return type.
	<all_comp_units>: Use dwarf2_per_cu_data_up.
	* dwarf2/read.c (dwarf2_per_cu_data::operator()): New function.
	(dwarf2_per_bfd::allocate_per_cu): Return dwarf2_per_cu_data_up.
	(create_cu_from_index_list): Likewise.
	(create_signatured_type_table_from_index)
	(create_cus_from_debug_names_list, add_type_unit)
	(read_comp_units_from_section): Update.
	(dwarf2_find_containing_comp_unit): Change type of all_comp_units.
	(run_test): Update.