gdbsupport/intrusive-list: make insert return an iterator

Make the insert method return an iterator to the inserted element.  This
mimics what boost does [1] and what the standard library insert methods
generally do [2].

[1] https://www.boost.org/doc/libs/1_79_0/doc/html/boost/intrusive/list.html#idm33771-bb
[2] https://en.cppreference.com/w/cpp/container/vector/insert



Change-Id: I59082883492c60ee95e8bb29a18c9376283dd660
Reviewed-by: Keith Seitz <keiths@redhat.com>