Use proper set operations for symtab dupe checks

Use proper set operations for symtab dupe checks

In query_symtab_func_info, rather than full set iteration to check an
address in alias_dupes, just use set::insert().second as a test.  This
is what sets are designed to be algorithmically good at.

This also has the benefit of adding to alias_dupes, so duplicates within
the symbol table itself will still only be probed once.  (If we didn't
want that effect, we would just use set::count() to test membership.)