In RelBuilder, simplify "CAST(literal TO type)" to a literal when possible

In CALCITE-1357 we added some logic (in DruidDateTimeUtils.leafToRanges) to recognize various combinations of literals, casts of literals and comparison operators, and I feel that logic would be simpler if we moved the logic into RexUtil and flattened the casts of literals to just plain literals. jcamachorodriguez, what do you think?