Allow calling a private method with self.

This makes it consistent with calling private attribute assignment
methods, which currently is allowed (e.g. `self.value =`).

Calling a private method in this way can be useful when trying to
assign the return value to a local variable with the same name.

[Feature #11297] [Feature #16123]

test/ruby/test_module.rb

@@ -2479,7 +2479,8 @@ class TestModule < Test::Unit::TestCase
       assert_include(methods, :#{method}, ":#{method} should be private")
 
       assert_raise_with_message(NoMethodError, "private method `#{method}' called for main:Object") {
-        self.#{method}
+        recv = self
+        recv.#{method}
       }
     }
   end