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

0 files changed, 0 insertions, 0 deletions