fix 21756 - make cast(immutable)arrayLiteral @nogc

[ghost]

adjust array literal semantics so that

cast(immutable) <arrayLiteral>

gets rewritten to a DeclExp comma VarExp

(static immutable __array = <arrayLiteral>, __array)

allowing immutable array literals to be @nogc

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	21756	major	Immutable array literals cause runtime GC allocation instead of static readonly section allocation

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#12329"

[Geod24]

I don't think that's a good thing. It's too much magic with no justification. As commented on the issue, using static immutable directly achieves what the user want.

[ghost]

This breaks legit code anyway. When the elements are not statically evaluable, you can stll put them in an array literal and cast this array as immutable. That would break. That stuff is never done in the test suite as you can see.