Warn duplication of __ENCODING__ on the hash

```
$ ruby -e 'h = { __ENCODING__ => 1, __ENCODING__ => 2 }'
-e:1: warning: key #<Encoding:UTF-8> is duplicated and overwritten on line 1
```

parse.y

@@ -90,6 +90,7 @@ hash_literal_key_p(VALUE k)
           case NODE_SYM:
           case NODE_LINE:
           case NODE_FILE:
+          case NODE_ENCODING:
             return true;
           default:
             return false;
@@ -193,6 +194,8 @@ node_cdhash_cmp(VALUE val, VALUE lit)
             return node_val->nd_loc.beg_pos.lineno != node_lit->nd_loc.beg_pos.lineno;
           case NODE_FILE:
             return rb_parser_string_hash_cmp(RNODE_FILE(node_val)->path, RNODE_FILE(node_lit)->path);
+          case NODE_ENCODING:
+            return RNODE_ENCODING(node_val)->enc != RNODE_ENCODING(node_lit)->enc;
           default:
             rb_bug("unexpected node: %s, %s", ruby_node_name(type_val), ruby_node_name(type_lit));
         }
@@ -15461,6 +15464,7 @@ nd_type_st_key_enable_p(NODE *node)
       case NODE_SYM:
       case NODE_LINE:
       case NODE_FILE:
+      case NODE_ENCODING:
         return true;
       default:
         return false;

test/ruby/test_literal.rb

@@ -498,6 +498,7 @@ class TestRubyLiteral < Test::Unit::TestCase
       '//',
       '__LINE__',
       '__FILE__',
+      '__ENCODING__',
     ) do |key|
       assert_warning(/key #{Regexp.quote(eval(key).inspect)} is duplicated/) { eval("{#{key} => :bar, #{key} => :foo}") }
     end