Produce a better error for irrefutable if let patterns

Modify ast::ExprMatch to include a new value of type ast::MatchSource,
making it easy to tell whether the match was written literally or
produced via desugaring. This allows us to customize error messages
appropriately.

src/libsyntax/fold.rs

@@ -1226,9 +1226,10 @@ pub fn noop_fold_expr<T: Folder>(Expr {id, node, span}: Expr, folder: &mut T) ->
                 ExprLoop(folder.fold_block(body),
                         opt_ident.map(|i| folder.fold_ident(i)))
             }
-            ExprMatch(expr, arms) => {
+            ExprMatch(expr, arms, source) => {
                 ExprMatch(folder.fold_expr(expr),
-                        arms.move_map(|x| folder.fold_arm(x)))
+                        arms.move_map(|x| folder.fold_arm(x)),
+                        source)
             }
             ExprFnBlock(capture_clause, decl, body) => {
                 ExprFnBlock(capture_clause,