[trivial] fix issue 18837 - MMFile should have opDollar

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[dlang-bot]

Thanks for your pull request and interest in making D better, @bbasile! 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 annotated coverage diff directly on GitHub with CodeCov's browser extension
• 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
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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	18837	enhancement	MMFile should have opDollar

Testing this PR locally

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

dub fetch digger
dub run digger -- build "master + phobos#6521"