* 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