Fix Bugzilla 12885 - const union wrongly converts implicitly to mutable

[ntrel]

This disables code that breaks immutable which was added for https://issues.dlang.org/show_bug.cgi?id=11257#c3, for -preview=fixImmutableConv only.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	12885	major	const union wrongly converts implicitly to mutable

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#16594"

[thewilsonator]

should this go to stable?

[RazvanN7]

I think this is fine in master since it's very old.