[Support] Redefine endianness::native (#67876)

We should eventually migrate llvm::support::endianness to std::endian
when C++20 is available for the codebase.

This patch prepares our codebase for that future by removing the
assumption that native is a unique value different from both big and
little.  Note that in C++20, native is equal to either big or little
on typical machines, where the endianness is the same for all scalar
types.