Implement D1957R0, T* to bool should be considered narrowing.

Implement D1957R0, T* to bool should be considered narrowing.

This paper was delayed until the February meeting in Prague so that we could
get a better idea of what the impact on existing code would actually be.  To
that end, I'm implementing it now.

	* typeck2.c (check_narrowing): Treat pointer->bool as a narrowing
	conversion with -std=c++2a.

From-SVN: r278026