Refactor 'expressionSemanticDone' logic into a function

[dkorpel]

Following up #20594

I see now the issue is not really about expressionSemantic not being idempotent, but about the Expression.type field sometimes being used to assign a type rather than to compute it. I think there should be a semanticDone bitflag in Expression since the type field doubling as a semantic status field is not consistent, but my first naive attempt resulted in segfaults, so let's start by documenting the current behavior more accurately.

[dlang-bot]

Thanks for your pull request and interest in making D better, @dkorpel! 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 + dmd#20600"