Fix issue 11725: AA.dup should return mutable AA where possible

[ntrel]

Reboot of #2838.

• Keeps same key type as I don't think there's any need to make that mutable.
• Changes a unittest to a versioned function as it doesn't do any runtime tests.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	11725	normal	[AA] Cannot dup const AA

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#16979"

[ntrel]

runnable/testaa3.d(118): Error: cannot cast [5:4] to int[int] at compile time

I don't know why that happens.