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/ast.rs

@@ -336,6 +336,7 @@ impl Default for Generics {
             where_clause: WhereClause {
                 id: DUMMY_NODE_ID,
                 predicates: Vec::new(),
+                span: DUMMY_SP,
             },
             span: DUMMY_SP,
         }
@@ -347,6 +348,7 @@ impl Default for Generics {
 pub struct WhereClause {
     pub id: NodeId,
     pub predicates: Vec<WherePredicate>,
+    pub span: Span,
 }
 
 /// A single predicate in a `where` clause