[BOLT][NFC] Use std::optional for findAttributeInfo

LLVM started switching from `llvm::Optional` to `std::optional`:
https://discourse.llvm.org/t/deprecating-llvm-optional-x-hasvalue-getvalue-getvalueor/63716/11

Reviewed By: #bolt, maksfb

Differential Revision: https://reviews.llvm.org/D139259