Change std::ceil2 to be undefined if the result can't be represented

Change std::ceil2 to be undefined if the result can't be represented

	* include/std/bit (__ceil2): Make unrepresentable results undefined,
	as per P1355R2. Add debug assertion. Perform one left shift, not two,
	so that out of range values cause undefined behaviour. Ensure that
	shift will still be undefined if left operand is promoted.
	* testsuite/26_numerics/bit/bit.pow.two/ceil2.cc: Replace checks for
	unrepresentable values with checks that they are not core constant
	expressions.
	* testsuite/26_numerics/bit/bit.pow.two/ceil2_neg.cc: New test.

From-SVN: r273705