comparison: Fix index error when interpreting scope comparison

comparison: Fix index error when interpreting scope comparison

While looking at something else, I noticed a thinko in the
code in scope_diff::ensure_lookup_tables_populated that interprets the
result of the diffing algorithm on the decls of a given scope.  Fixed
thus.

	* src/abg-comparison.cc
	(scope_diff::ensure_lookup_tables_populated): Use the proper index
	to address the deleted decl.  Also, use the range-based for syntax
	in the enclosing for-loop for more clarity.

Signed-off-by: Dodji Seketeli <dodji@redhat.com>