Fixed the cas function not returning the result

[TurkeyMan]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	20122	enhancement	core.atomic.cas discards result on failure

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 "master + druntime#2733"