Improve error message for non-exhaustive matches on non-exhaustive enums

[FabianWolff]

This pull request fixes #85227. For an enum marked with #[non_exhaustive] and not defined in the current crate, the error message for non-exhaustive matches now mentions the fact that the enum is marked as non-exhaustive:

error[E0004]: non-exhaustive patterns: `_` not covered
  --> main.rs:12:11
   |
12 |     match e {
   |           ^ pattern `_` not covered
   |
   = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
   = note: the matched value is of type `E`, which is marked as non-exhaustive

[rust-highfive]

r? @petrochenkov

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[petrochenkov]

@bors r+

[bors]

📌 Commit 57291b8 has been approved by petrochenkov

[bors]

⌛ Testing commit 57291b8 with merge 69b352e...

[bors]

☀️ Test successful - checks-actions
Approved by: petrochenkov
Pushing 69b352e to master...