Convert a bool to Trailing.

This pre-existing type is suitable for use with the return value of the
`f` parameter in `collect_tokens_trailing_token`. The more descriptive
name will be useful because the next commit will add another boolean
value to the return value of `f`.

compiler/rustc_parse/src/parser/pat.rs

@@ -1318,7 +1318,7 @@ impl<'a> Parser<'a> {
                     last_non_comma_dotdot_span = Some(this.prev_token.span);
 
                     // We just ate a comma, so there's no need to capture a trailing token.
-                    Ok((field, false))
+                    Ok((field, Trailing::No))
                 })?;
 
             fields.push(field)