[stable] Cherry-pick changes from master that are needed for dub to compile

[Geod24]

Dub uses -preview=in to compile in its build.d script, but not in the dub.sdl (because Vibe.d wasn't DIP1000 compatible).
DMD v2.101.0 introduced a change that means -preview=in now disallows in on non-extern(D) / extern(C++) functions.
But since the fixes were done 3 days after the start of the v2.101 release, the dub CI is currently broken with latest.

[dlang-bot]

Thanks for your pull request, @Geod24!

Bugzilla references

Your PR doesn't reference any Bugzilla issue.

If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#8627"

[ibuclaw]

@Geod24

1. Try not to cherry pick, commit to stable then merge to master to avoid conflicts and wasted time fixing them up.
2. Title says stable, but you target master.

[Geod24]

Try not to cherry pick, commit to stable then merge to master to avoid conflicts and wasted time fixing them up.

git detect this quite well actually, at least from my experience when rebasing. What's your preferred approach ?

Title says stable, but you target master.

🤦‍♂️