Improve handling of spans around macro result parse errors

Fixes #81543

After we expand a macro, we try to parse the resulting tokens as a AST
node. This commit makes several improvements to how we handle spans when
an error occurs:

* Only ovewrite the original `Span` if it's a dummy span. This preserves
  a more-specific span if one is available.
* Use `self.prev_token` instead of `self.token` when emitting an error
  message after encountering EOF, since an EOF token always has a dummy
  span
* Make `SourceMap::next_point` leave dummy spans unused. A dummy span
  does not have a logical 'next point', since it's a zero-length span.
  Re-using the span span preserves its 'dummy-ness' for other checks

compiler/rustc_expand/src/expand.rs

@@ -896,7 +896,9 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
                 fragment
             }
             Err(mut err) => {
-                err.set_span(span);
+                if err.span.is_dummy() {
+                    err.set_span(span);
+                }
                 annotate_err_with_kind(&mut err, kind, span);
                 err.emit();
                 self.cx.trace_macros_diag();