gdb: change inf_threads_iterator to yield references

gdb: change inf_threads_iterator to yield references

When adding reference_to_pointer_iterator, I saw it as a temporary
thing, to not have to do a codebase-wide change right away.  Remove it
from inf_threads_iterator and adjust all the users.

It's very possible that I forgot to update some spots in the files I
can't compile, but it will be very easy to fix if that happens.

Change-Id: Iddc462fecfaafb6a9861d185b217bc714e7dc651
Approved-By: Tom Tromey <tom@tromey.com>