Make DustMite work with -preview=fieldwise

[pbackus]

Needed for dlang/dmd#20670

[dlang-bot]

Thanks for your pull request and interest in making D better, @pbackus! 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.

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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 + tools#477"

[pbackus]

generated/linux/64/rdmd_test generated/linux/64/rdmd -m64 \
   --rdmd-default-compiler=dmd \
   --test-compilers=dmd \
   
object.Exception@rdmd_test.d(143): Enforcement failed

Looks like CI for this repository is completely broken. I can reproduce this error locally on master.

[pbackus]

Closing in favor of dlang/dmd#20672