Fix issue 18504 - Generate unwind tables for halting functions

[LemonBoy]

I think #7333 heuristic is too eager.

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	18504	major	Assert in synchronized crashes with SIGILL on exit

[JinShil]

LGTM, but I'm not really qualified. @WalterBright or @ibuclaw, please review.

[WalterBright]

When creating a PR to fix a bugzilla issue, please post a link to the PR in the issue. This is so that someone browsing bug reports will know if and where a PR for it is.

[ibuclaw]

Why is this information in the frontend? This is a backend check, and dmd specific.

[WalterBright]

All it does is set a flag, which is then used by the glue code and back end.