Overhaul how stashed diagnostics work, again.

Stashed errors used to be counted as errors, but could then be
cancelled, leading to `ErrorGuaranteed` soundness holes. #120828 changed
that, closing the soundness hole. But it introduced other difficulties
because you sometimes have to account for pending stashed errors when
making decisions about whether errors have occured/will occur and it's
easy to overlook these.

This commit aims for a middle ground.
- Stashed errors (not warnings) are counted immediately as emitted
  errors, avoiding the possibility of forgetting to consider them.
- The ability to cancel (or downgrade) stashed errors is eliminated, by
  disallowing the use of `steal_diagnostic` with errors, and introducing
  the more restrictive methods `try_steal_{modify,replace}_and_emit_err`
  that can be used instead.

Other things:
- `DiagnosticBuilder::stash` and `DiagCtxt::stash_diagnostic` now both
  return `Option<ErrorGuaranteed>`, which enables the removal of two
  `delayed_bug` calls and one `Ty::new_error_with_message` call. This is
  possible because we store error guarantees in
  `DiagCtxt::stashed_diagnostics`.
- Storing the guarantees also saves us having to maintain a counter.
- Calls to the `stashed_err_count` method are no longer necessary
  alongside calls to `has_errors`, which is a nice simplification, and
  eliminates two more `span_delayed_bug` calls and one FIXME comment.
- Tests are added for three of the four fixed PRs mentioned below.
- `issue-121108.rs`'s output improved slightly, omitting a non-useful
  error message.

Fixes #121451.
Fixes #121477.
Fixes #121504.
Fixes #121508.

compiler/rustc_passes/src/check_attr.rs

@@ -2520,14 +2520,6 @@ fn check_invalid_crate_level_attr(tcx: TyCtxt<'_>, attrs: &[Attribute]) {
         if attr.style == AttrStyle::Inner {
             for attr_to_check in ATTRS_TO_CHECK {
                 if attr.has_name(*attr_to_check) {
-                    if let AttrKind::Normal(ref p) = attr.kind
-                        && let Some(diag) = tcx.dcx().steal_diagnostic(
-                            p.item.path.span,
-                            StashKey::UndeterminedMacroResolution,
-                        )
-                    {
-                        diag.cancel();
-                    }
                     let item = tcx
                         .hir()
                         .items()
@@ -2537,7 +2529,7 @@ fn check_invalid_crate_level_attr(tcx: TyCtxt<'_>, attrs: &[Attribute]) {
                             span: item.ident.span,
                             kind: item.kind.descr(),
                         });
-                    tcx.dcx().emit_err(errors::InvalidAttrAtCrateLevel {
+                    let err = tcx.dcx().create_err(errors::InvalidAttrAtCrateLevel {
                         span: attr.span,
                         sugg_span: tcx
                             .sess
@@ -2553,6 +2545,16 @@ fn check_invalid_crate_level_attr(tcx: TyCtxt<'_>, attrs: &[Attribute]) {
                         name: *attr_to_check,
                         item,
                     });
+
+                    if let AttrKind::Normal(ref p) = attr.kind {
+                        tcx.dcx().try_steal_replace_and_emit_err(
+                            p.item.path.span,
+                            StashKey::UndeterminedMacroResolution,
+                            err,
+                        );
+                    } else {
+                        err.emit();
+                    }
                 }
             }
         }