Tweak error counting.

We have several methods indicating the presence of errors, lint errors,
and delayed bugs. I find it frustrating that it's very unclear which one
you should use in any particular spot. This commit attempts to instill a
basic principle of "use the least general one possible", because that
reflects reality in practice -- `has_errors` is the least general one
and has by far the most uses (esp. via `abort_if_errors`).

Specifics:
- Add some comments giving some usage guidelines.
- Prefer `has_errors` to comparing `err_count` to zero.
- Remove `has_errors_or_span_delayed_bugs` because it's a weird one: in
  the cases where we need to count delayed bugs, we should really be
  counting lint errors as well.
- Rename `is_compilation_going_to_fail` as
  `has_errors_or_lint_errors_or_span_delayed_bugs`, for consistency with
  `has_errors` and `has_errors_or_lint_errors`.
- Change a few other `has_errors_or_lint_errors` calls to `has_errors`,
  as per the "least general" principle.

This didn't turn out to be as neat as I hoped when I started, but I
think it's still an improvement.

compiler/rustc_middle/src/ty/visit.rs

@@ -55,10 +55,13 @@ pub trait TypeVisitableExt<'tcx>: TypeVisitable<TyCtxt<'tcx>> {
     }
     fn error_reported(&self) -> Result<(), ErrorGuaranteed> {
         if self.references_error() {
-            if let Some(reported) = ty::tls::with(|tcx| tcx.dcx().is_compilation_going_to_fail()) {
+            // We must include lint errors and span delayed bugs here.
+            if let Some(reported) =
+                ty::tls::with(|tcx| tcx.dcx().has_errors_or_lint_errors_or_delayed_bugs())
+            {
                 Err(reported)
             } else {
-                bug!("expect tcx.sess.is_compilation_going_to_fail return `Some`");
+                bug!("expected some kind of error in `error_reported`");
             }
         } else {
             Ok(())