Pick up comments before where clause in traits (#4213)

Author: ayazhafiz

Diff for: rustfmt-core/rustfmt-lib/src/formatting/items.rs

@@ -1126,7 +1126,7 @@ pub(crate) fn format_trait(
 
             let where_budget = context.budget(last_line_width(&result));
             let pos_before_where = if generic_bounds.is_empty() {
-                generics.where_clause.span.lo()
+                generics.span.hi()
             } else {
                 generic_bounds[generic_bounds.len() - 1].span().hi()
             };

Diff for: rustfmt-core/rustfmt-lib/tests/target/issue-4211.rs

@@ -0,0 +1,13 @@
+trait SomeTrait<T>
+// comment
+where
+    T: Copy,
+{
+}
+
+trait SomeTrait
+// comment
+where
+    T: Copy,
+{
+}