dwarf-reader: get_die_source: always initialize return value

dwarf-reader: get_die_source: always initialize return value

GCC9 with ABIGAIL_DEVEL=1 and ABIGAIL_DEBUG=1 set, regularly emits
-Werror=maybe-uninitialized for the values gathered by get_die_source.
As a counter measure, some of them were initialized before the call to
NO_DEBUG_INFO_DIE_SOURCE, but not all of them, leading to said warning.
In order to overcome this, let get_die_source always initialize the
source to NO_DEBUG_INFO_DIE_SOURCE and adjust the caller to consistently
not do that anymore. This solves the warning and maybe but unlikely (due
to the ABG_ASSERT) avoids some UB.

Changing the interface of get_die_source to return the source directly,
lead to the ability to initialize the value const and to defer the
assertion to the function itself. Some occurrences could be removed
entirely as the die_source was not used at all.

	* src/abg-dwarf-reader.cc
	(read_context::get_die_source): Always initialize die_source.
	(read_context::ContainerType::get_container): Fix
	initialization of die_source.
	(read_context::compute_canonical_die): Likewise.
	(read_context::get_canonical_die): Likewise.
	(read_context::get_or_compute_canonical_die): Likewise.
	(read_context::associate_die_to_decl): Likewise.
	(read_context::set_canonical_die_offset): Likewise.
	(read_context::schedule_type_for_late_canonicalization): Likewise.
	(read_context::compare_dies): Likewise.
	(read_context::get_parent_die): Likewise.
	(read_context::get_scope_for_die): Likewise.
	(read_context::add_or_update_union_type): Likewise.
	(read_context::maybe_canonicalize_type): Likewise.
	(read_context::build_ir_node_from_die): Likewise.

Signed-off-by: Matthias Maennich <maennich@google.com>