Fix issue 17918 - Stop if the late semantic pass fails

[LemonBoy]

No matter what the outcome of finishVtbl was the old code kept building
the rest of the class. If the late semantic pass over the
FunctionDeclaration fails we should abort the codegen phase instead of
going on.

[dlang-bot]

Thanks for your pull request and interest in making D better, @LemonBoy! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	17918	critical	[Reg 2.072] ICE with unknown symbol in base class

⚠️⚠️⚠️ Warnings ⚠️⚠️⚠️

• Regression or critical bug fixes should always target the stable branch. Learn more about rebasing to stable or the D release process.

To target stable perform these two steps:

1. Rebase your branch to upstream/stable:

git rebase --onto upstream/stable upstream/master

2. Change the base branch of your PR to stable

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 + dmd#8096"

[WalterBright]

Use standard Ddoc Returns: form.