Adjusted diagnostic output so that if there is no use in a item sequence,

then we just suggest the first legal position where you could inject a use.

To do this, I added `inject_use_span` field to `ModSpans`, and populate it in
parser (it is the span of the first token found after inner attributes, if any).
Then I rewrote the use-suggestion code to utilize it, and threw out some stuff
that is now unnecessary with this in place. (I think the result is easier to
understand.)

Then I added a test of issue 87613.

src/parse/parser.rs

@@ -113,7 +113,7 @@ impl<'a> Parser<'a> {
         let result = catch_unwind(AssertUnwindSafe(|| {
             let mut parser = new_parser_from_file(sess.inner(), path, Some(span));
             match parser.parse_mod(&TokenKind::Eof) {
-                Ok((a, i, ast::ModSpans { inner_span })) => Some((a, i, inner_span)),
+                Ok((a, i, ast::ModSpans { inner_span, inject_use_span: _ })) => Some((a, i, inner_span)),
                 Err(mut e) => {
                     e.emit();
                     if sess.can_reset_errors() {