Fix `ErrorGuaranteed` unsoundness with stash/steal.
When you stash an error, the error count is incremented. You can then use the non-zero error count to get an `ErrorGuaranteed`. You can then steal the error, which decrements the error count. You can then cancel the error.
Example code:
```
fn unsound(dcx: &DiagCtxt) -> ErrorGuaranteed {
let sp = rustc_span::DUMMY_SP;
let k = rustc_errors::StashKey::Cycle;
dcx.struct_err("bogus").stash(sp, k); // increment error count on stash
let guar = dcx.has_errors().unwrap(); // ErrorGuaranteed from error count > 0
let err = dcx.steal_diagnostic(sp, k).unwrap(); // decrement error count on steal
err.cancel(); // cancel error
guar // ErrorGuaranteed with no error emitted!
}
```
This commit fixes the problem in the simplest way: by not counting stashed errors in `DiagCtxt::{err_count,has_errors}`.
However, just doing this without any other changes leads to over 40 ui test failures. Mostly because of uninteresting extra errors (many saying "type annotations needed" when type inference fails), and in a few cases, due to delayed bugs causing ICEs when no normal errors are printed.
To fix these, this commit adds `DiagCtxt::stashed_err_count`, and uses it in three places alongside `DiagCtxt::{has_errors,err_count}`. It's dodgy to rely on it, because unlike `DiagCtxt::err_count` it can go up and down. But it's needed to preserve existing behaviour, and at least the three places that need it are now obvious.
r? oli-obk
2f1ac412ec