Compute Current Time* expressions should be foldable

Following PR - https://github.com/apache/spark/pull/44261 changed "compute current time" family of expressions to be unevaluable, given that these expressions are supposed to be replaced with literals by QO. Unevaluable implies that these expressions are not foldable, even though they will be replaced by literals.

If these expressions were used in places that require constant folding (e.g. RAND()) new behavior would be to raise an error which is a regression comparing to behavior prior to spark 4.0.