[Support, ADT] Move llvm::endianness to bit.h (#68280)

In C++20, std::endian comes from `<bit>`.  Following the same spirit,
this patch moves llvm::endianness and the endian detection logic to
bit.h.

Without this patch, llvm::endianness::native is defined in terms of
llvm::sys::IsBigEndianHost.  This patch reverses the dependency.
llvm::sys::IsBigEndianHost is now defined in terms of
llvm::endianness::native.