[polly] Use std::optional instead of llvm::Optional (NFC)

This is part of an effort to migrate from llvm::Optional to
std::optional:

https://discourse.llvm.org/t/deprecating-llvm-optional-x-hasvalue-getvalue-getvalueor/63716

GitOrigin-RevId: ccdc271a0899c2225a9cd2db1af61c1d5cf8fcad
6 files changed