Simplify std::rethrow_if_nested definition

Simplify std::rethrow_if_nested definition

	* doc/xml/manual/intro.xml: Document LWG 2484 status.
	* libsupc++/nested_exception.h (_Throw_with_nested_impl)
	(_Throw_with_nested_helper): Remove.
	(__throw_with_nested_impl): New overloaded functions to implement
	throw_with_nested logic.
	(throw_with_nested): Call __throw_with_nested_impl.
	(_Rethrow_if_nested_impl): Remove
	(__rethrow_if_nested_impl): New overloaded functions to implement
	rethrow_if_nested logic.
	(rethrow_if_nested): Call __rethrow_if_nested_impl.
	* testsuite/18_support/nested_exception/rethrow_if_nested.cc: Test
	problem cases from LWG 2484.

From-SVN: r240983