Improve E0391

[estebank]

In #60126 we continue compiling on what used to be a hard error, causing both E0072 (recursive type) and E0391 (cycle detected when processing). In this case in particular, the later error shouldn't be emitted if it is already explained by another error. At the same time, the current output is less than ideal and could be improved:

error[E0391]: cycle detected when processing `MList`
  --> $DIR/infinite-tag-type-recursion.rs:1:1
   |
LL | enum MList { Cons(isize, MList), Nil }
   | ^^^^^^^^^^
   |
   = note: ...which again requires processing `MList`, completing the cycle
   = note: cycle used when computing dropck types for `Canonical { max_universe: U0, variables: [], value: ParamEnvAnd { param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, def_id: None }, value: MList } }`

[estebank]

Current output

error[E0072]: recursive type `MList` has infinite size
 --> src/lib.rs:1:1
  |
1 | enum MList { Cons(isize, MList), Nil }
  | ^^^^^^^^^^               ----- recursive without indirection
  |
help: insert some indirection (e.g., a `Box`, `Rc`, or `&`) to break the cycle
  |
1 | enum MList { Cons(isize, Box<MList>), Nil }
  |                          ++++     +