c++: Implement P1957R2, T* to bool should be considered narrowing.

c++: Implement P1957R2, T* to bool should be considered narrowing.

This was approved in the Prague 2020 WG21 meeting so let's adjust the
comment.  Since it's supposed to be a DR I think we should no longer
limit it to C++20.

	P1957R2
	* typeck2.c (check_narrowing): Consider T* to bool narrowing
	in C++11 and up.

	* g++.dg/cpp0x/initlist92.C: Don't expect an error in C++20 only.