[REVIEW REQUEST] CALCITE-5230 PERCENTILE_DISC return type derivation

[Itiel Sadeh]

Hello,
I would like to request review for CALCITE-5230, which fixes the return
type derivation of PERCENTILE_DISC.

PR: https://github.com/apache/calcite/pull/2868
Ticket: https://issues.apache.org/jira/browse/CALCITE-5230

Thanks!