Fix issue 20354: interface is not supported by cas

[lempiji]

I want to fix this issue.
Some packages are breaking now. 😢

https://issues.dlang.org/show_bug.cgi?id=20354

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	20354	regression	interface is not supported by CanCAS in core.internal.atomic

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub fetch digger
dub run digger -- build "stable + druntime#2853"

[thewilsonator]

src\core\atomic.d(246): Error: undefined identifier `U`
src\core\atomic.d(830): Error: template instance `core.atomic.atomicExchange!(cast(MemoryOrder)5, Inter, shared(Inter))` error instantiating
src\core\atomic.d(910):        instantiated from here: `testXCHG!(shared(Inter))`

I think https://github.com/dlang/druntime/blob/master/src/core/atomic.d#L245-L246 is a typo.

CC @TurkeyMan git blames you for this

Should that be alias Thunk = Unshared!V;?

[thewilsonator]

Also as this is a regression, please target stable.

[PetarKirov]

I rebased the PR to stable and fixed the remaining errors.
@TurkeyMan can you take another look?

[thewilsonator]

LGTM