c++: make -fpermissive avoid -Werror=narrowing

Currently we make -Wnarrowing an error by default by forcing pedantic_errors
on, but for consistency -fpermissive should prevent that.

In general I'm inclined to move away from using permerror in favor of this
kind of model, with specific flags for each diagnostic.

gcc/cp/ChangeLog:

	* typeck2.cc (check_narrowing): Check flag_permissive.