Auto merge of #27185 - pnkfelix:test-cyclic-collections, r=alexcrichton

This test attempts to exercise cyclic structure in much of `std::collections`

(as much as is possible; e.g. things like `EnumSet` and `BitVec` do not really support carrying references at all, so trying to represent cyclic structure within them dooes not really make sense.)

This work should land before we make future revisions to `dropck`; that is, I am attempting to catch potential regressions to cases where we supported cyclic structure circa Rust 1.2.

Author: bors