fix issue 20655 - attribute inference accepts unsafe union access as @safe

[dkorpel]

Reboot of #10884

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	20655	regression	[REG: 2.072] attribute inference accepts unsafe union access as @safe

⚠️⚠️⚠️ 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:

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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#14827"

[ibuclaw]

Well, that failed. :-)

[dkorpel]

It's now mostly working, but D-YAML, excel-d, taggedalgebraic, and serialized depend on inferred @safe despite union access, so I'll turn this into a deprecation.

[maxhaton]

This error message isn't great. The field isn't the thing doing the access

[dkorpel]

This PR isn't about the message