parse.y: else without rescue

* parse.y (bodystmt): [EXPERIMENTAL] make `else` without `rescue`
  a syntax error.  [DevelopersMeeting20180315Japan]

  https://bugs.ruby-lang.org/projects/ruby/wiki/DevelopersMeeting20180315Japan
  https://docs.google.com/document/d/1RT0ijSo8uJ4Awn3CEvuYkjH0TVeXSYgeAFNmVGYC3ak/edit#
  > * do-else-end
  >   https://twitter.com/joker1007/status/974173396006129664
  >   https://twitter.com/joker1007/status/974173641347756032
  >   https://twitter.com/joker1007/status/974176512554369027
  >   Will be SyntaxError in 2.6-preview2
  >   All of begin/do/def (experimental)

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@62901 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/ruby/test_parse.rb

@@ -14,13 +14,12 @@ class TestParse < Test::Unit::TestCase
   end
 
   def test_else_without_rescue
-    x = eval <<-END, nil, __FILE__, __LINE__+1
+    assert_syntax_error(<<-END, /else without rescue/)
       begin
       else
         42
       end
     END
-    assert_equal(42, x)
   end
 
   def test_alias_backref