Undefine allocator for Polyglot::ForeignException

* The same way as for Integer, Symbol, etc.
* Polyglot::ForeignObject.new would otherwise return a RubyBasicObject
  which is not actually a foreign object, and that would lead to confusion.
* #new is overridden by InstantiableTrait and that works fine for
  InteropLibrary#isInstantiable() foreign objects.

Author: nirvdrum

src/main/ruby/truffleruby/core/truffle/polyglot.rb

@@ -546,6 +546,14 @@ def self.__allocate__
   class ForeignException < Exception # rubocop:disable Lint/InheritException
     include ObjectTrait
     include ExceptionTrait
+
+    class << self
+      undef_method :new
+    end
+
+    def self.__allocate__
+      raise TypeError, "allocator undefined for #{self}"
+    end
   end
   # endregion
 end