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_span/src/source_map.rs

@@ -799,6 +799,9 @@ impl SourceMap {
 
     /// Returns a new span representing the next character after the end-point of this span.
     pub fn next_point(&self, sp: Span) -> Span {
+        if sp.is_dummy() {
+            return sp;
+        }
         let start_of_next_point = sp.hi().0;
 
         let width = self.find_width_of_character_at_span(sp.shrink_to_hi(), true);