[simd] Resolve LWG4420 and LWG4393

[mattkretz]

This provides the proposed resolution to LWG4420, which then also resolves LWG4393.

[jensmaurer]

This pull request has nothing to do with the current proposed resolution on the mentioned LWG issues.
Also, where did the "output_range" constraint go that would address LWG4393?
And please use the correct git commit title per https://github.com/cplusplus/draft/wiki/Commit-message-format .
And we probably have an overfull \hbox or so; the CI build is failing.

[mattkretz]

Sorry @jensmaurer, I did not expect a review as long as the PR is in draft status.
I just sent a big update to LWG4420 and wanted to point to a PR from it, which is why I opened it here. I'll fix the hboxes and commit message.

[mattkretz]

this is outdated and would need a new PR from my new fork anyway

[tkoeppe]

All the forks have been restored -- could you have another look?

[mattkretz]

Yes, I can update it with the last changes. How does this work wrt. #8461? (It's one part of LWG Motion 2)