Rollup merge of #43501 - topecongiro:span-to-whereclause, r=nrc

Add Span to ast::WhereClause

This PR adds `Span` field to `ast::WhereClause`. The motivation here is to make rustfmt's life easier when recovering comments before and after where clause.
r? @nrc

src/libsyntax/parse/mod.rs

@@ -892,6 +892,7 @@ mod tests {
                                         where_clause: ast::WhereClause {
                                             id: ast::DUMMY_NODE_ID,
                                             predicates: Vec::new(),
+                                            span: syntax_pos::DUMMY_SP,
                                         },
                                         span: syntax_pos::DUMMY_SP,
                                     },