The distinct non-null values of values, in order, as an independent copy.
Same distinctness rule as pf_unique_count -- exact for reals, all NaNs collapsing to
one. descending chooses the order the distinct values come back in; there is no
nulls_first, because nulls are excluded rather than placed.
Each distinct value is taken from its FIRST occurrence in the sorted order, which for
equal-comparing-but-not-identical values (a character array's trailing blanks, a
parquet_string_column's empty strings) is the earliest such element of values.