Implement most of P1612R1: Relocate endian. Moves the std::endian...

Implement most of P1612R1: Relocate endian.  Moves the std::endian functionality from 'type-traits' to 'bit'. No other change. The reason that this is 'partial' is that P1621 also recommends a feature-test macro, but I don't have the value for that one yet. In a month or so, I'll add that

llvm-svn: 366776