Annotate std/file.d to please dlang/dmd#12520

[nordlow]

Part of #8076

The #8107 and this PR are the two remaining Phobos blockers for dlang/dmd#12520.

[dlang-bot]

Thanks for your pull request and interest in making D better, @nordlow! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
Please verify that your PR follows this checklist:

• My PR is fully covered with tests (you can see the coverage diff by visiting the details link of the codecov check)
• My PR is as minimal as possible (smaller, focused PRs are easier to review than big ones)
• I have provided a detailed rationale explaining my changes
• New or modified functions have Ddoc comments (with Params: and Returns:)

Please see CONTRIBUTING.md for more information.

---

If you have addressed all reviews or aren't sure how to proceed, don't hesitate to ping us with a simple comment.

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#8085"

[nordlow]

make -f posix.mak unittest pass now using dlang/dmd#12520. Yay! Thanks, @MrSmith33.

Removed WIP in title. Squashed. Ready for review and auto-merge now.

Ping, @RazvanN7 @thewilsonator @dkorpel

[nordlow]

Does anybody know why Ubuntu 16.04 x86 and macOS 10.15 x64 fails? Restart?

[RazvanN7]

I restarted the failing tests. Let's see.