[llvm] [ADT] Use CMake to detect host endianness (NFC) (PR #164054)

[Kazu Hirata via llvm-commits]

kazutakahirata wrote:

@kuhar Shall we scrap this PR?  While we could clean up the existing code with CMake's endian detection, we'll probably move to C++20 within one or two years.  (We've switched to C++17 on August 5, 2022 b1356504e63ae821cccf1e051a0d2526bdfef2b0) Once we adopt C++20, we can easily replace the whole block with `using endianness = std::endian;` to accommodate gradual migration in an arbitrary order without touching any other file.  That is, garbage collection is much easier than touching this file and removing the gadgets that this PR is trying to add.


https://github.com/llvm/llvm-project/pull/164054