[BUG][AUDIT][SPARK-50377][SQL] Allow to evaluate foldable RuntimeReplaceable

[abellina]

UnaryPositive is no longer a UnaryExpression (but we have it as a GpuUnaryExpression). It is now a RuntimeReplaceable.

This change was introduced earlier in Spark 4 but it causes issues because UnaryPositive stopped being foldable. In the cast of lag (for window definitions), for example, we can have UnaryPositive in the lag definition as lag(x, +5), and it stopped working after the change to RuntimeReplaceable.

With apache/spark@0fbd34c45b5#diff-68100e5e6079666747882f5070767601087afc8181f513d997f6e39a948eaa0dR1063, RuntimeReplaceable can now be evaluated.

So I am not 100% sure how much of this we care about. I think we need to possibly move to RuntimeReplaceable for UnaryPositive (and possibly others?) for Spark 4. That said, filing this because I think we should investigate this a bit to see what needs to change/what is broken.

[revans2]

RuntimeReplaceable gets replaced in logical plan translation and we never even see them by the time our code runs. At least that is the theory anyways. I would expect our code to still work so long as we can still compile. It just means that the replacement code will never run.