c++: Don't hide narrowing errors in system headers

c++: Don't hide narrowing errors in system headers

Jonathan pointed me at this issue where

  constexpr unsigned f() { constexpr int n = -1; return unsigned{n}; }

is accepted in system headers, despite the narrowing conversion from
a constant.  I suspect that whereas narrowing warnings should be
disabled, ill-formed narrowing of constants should be a hard error
(which can still be disabled by -Wno-narrowing).

gcc/cp/ChangeLog:

	* typeck2.c (check_narrowing): Don't suppress the pedantic error
	in system headers.

libstdc++-v3/ChangeLog:

	* testsuite/20_util/ratio/operations/ops_overflow_neg.cc: Add
	dg-error.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/Wnarrowing2.C: New test.
	* g++.dg/cpp1y/Wnarrowing2.h: New test.