Use gdb::unordered_set in linespec_state

This patch changes linespec_state to use gdb::unordered_set.  This
simplifies the code a little and removes some manual management.  It
also replaces address_entry with a std::pair, which simplifies the
code even more; and since this is a private type, IMO it doesn't
reduce readability at all.

Approved-By: Simon Marchi <simon.marchi@efficios.com>