avoid attribute ambiguity in ExprStmt

Both expr stmt and expr can have attributes, which doesn't make sense.
Let's say that statement's attrs are those of the expression.

Author: matklad

rust.ungram

@@ -318,7 +318,7 @@ LetStmt =
   '=' initializer:Expr ';'
 
 ExprStmt =
-  Attr* Expr ';'?
+  Expr ';'?
 
 Expr =
   ArrayExpr