Use [[no_unique_address]] after Clang supports it on Windows #1364

VS 2019 16.9 will support [[no_unique_address]]. Clang lists this as supported in Clang 9. However, we suspect that it is not yet supported when Clang targets Windows, because MSVC didn't support it. So, #1363 is commenting out all of our [[no_unique_address]] usage.

• Confirm whether Clang supports [[no_unique_address]] on Windows. (Clang does not.)
• If it does, proceed to the next step, otherwise we should file an LLVM bug requesting such support, then wait for it to be implemented and available in the version of Clang that ships with VS.
• Finally, we should begin using [[no_unique_address]] for C++20-only code.