E0391 needs to be updated to new format

[sophiajt]

From: src/test/compile-fail/issue-20825.rs

Error E0391 needs a span_label, updating it from:

error[E0391]: unsupported cyclic reference between types/traits detected
  --> src/test/compile-fail/issue-20825.rs:15:41
   |
15 | pub trait Processor: Subscriber<Input = Self::Input> {
   |                                         ^^^^^^^^^^^
   |
   = note: the cycle begins when computing the supertraits of `Processor`...
   = note: ...which then again requires computing the supertraits of `Processor`, completing the cycle.

To:

error[E0391]: unsupported cyclic reference between types/traits detected
  --> src/test/compile-fail/issue-20825.rs:15:41
   |
15 | pub trait Processor: Subscriber<Input = Self::Input> {
   |                                         ^^^^^^^^^^^ cyclic reference
   |
   = note: the cycle begins when computing the supertraits of `Processor`...
   = note: ...which then again requires computing the supertraits of `Processor`, completing the cycle.