In particular `LinkedList` and `RawTable` (the backing representation
for `HashMap` and `HashSet`).

This is to get the test from PR rust-lang#27185 building.

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.

I needed it in `RawVec`, `Vec`, and `TypedArena` for `rustc` to
bootstrap; but of course that alone was not sufficient for `make
check`.

Later I added `unsafe_destructor_blind_to_params` to collections, in
particular `LinkedList` and `RawTable` (the backing representation for
`HashMap` and `HashSet`), to get the regression tests exercising
cyclic structure from PR rust-lang#27185 building.

----

Note that the feature is `dropck_parametricity` (which is not the same
as the attribute's name). We will almost certainly vary our strategy
here in the future, so it makes some sense to have a not-as-ugly name
for the feature gate. (The attribute name was deliberately selected to
be ugly looking.)

Major revision to the dropck_legal_cycles test.

1. Added big comment block explaining the test framework.

2. Added tests exericising Rc and Arc. This was inspired by a comment
   from eefriedman on PR #28861.

3. Made the cycle-detection not issue false-positives on acyclic dags.

   Doing this efficiently required revising the framework; instead of
   visiting all children (i.e. doing a traversal), now each test is
   responsible for supplying the path that will act as a witness to
   the cycle.

   Luckily for me, all of the pre-existing tests worked with a trivial
   path built from "always tke your first left", but new tests I added
   did require other input paths (i.e., "first turn right, then left".

   (The path representation is a bit-string and its branches are
    n-ary, not word phrases and binary branches as you might think
    from the outline above.)

cc PR #27185

I needed it in `RawVec`, `Vec`, and `TypedArena` for `rustc` to
bootstrap; but of course that alone was not sufficient for `make
check`.

Later I added `unsafe_destructor_blind_to_params` to collections, in
particular `LinkedList` and `RawTable` (the backing representation for
`HashMap` and `HashSet`), to get the regression tests exercising
cyclic structure from PR rust-lang#27185 building.

----

Note that the feature is `dropck_parametricity` (which is not the same
as the attribute's name). We will almost certainly vary our strategy
here in the future, so it makes some sense to have a not-as-ugly name
for the feature gate. (The attribute name was deliberately selected to
be ugly looking.)