[variant.ctor] Add missing throws clause to last ctor

[Tsche]

Fixes #7751

[jensmaurer]

@Tsche , if you want your real name to appear in the git commit history, please configure it in the github profile settings.

@jwakely, does this need an LWG issue?

[jwakely]

I'll do some archaeology and see if it's been discussed or just forgotten (probably the latter)

[Tsche]

@Tsche , if you want your real name to appear in the git commit history, please configure it in the github profile settings.

Thanks. I'd prefer committing it under this pseudonym instead if that's okay with you.