E0446 needs to be updated to new format

[sophiajt]

From: src/test/compile-fail/E0446.rs

E0446 needs a span_label, updating it from:

error[E0446]: private type in public interface
  --> src/test/compile-fail/E0446.rs:14:21
   |
14 |     pub fn bar() -> Bar { //~ ERROR E0446
   |                     ^^^

To:

error[E0446]: private type in public interface
  --> src/test/compile-fail/E0446.rs:14:21
   |
14 |     pub fn bar() -> Bar { //~ ERROR E0446
   |                     ^^^ private type can't be public

[bstrie]

Freeing this one up as part of the final push for #35233.

[bstrie]

@Coder206 looks to have an in-progress PR to address this issue, you may want to base your work off of that one.