[clang][dataflow] Encode options using llvm::Optional

[clang][dataflow] Encode options using llvm::Optional

This patch restructures `DataflowAnalysisOptions` and `TransferOptions` to use `llvm::Optional`, in preparation for adding more sub-options to the `ContextSensitiveOptions` struct introduced here.

Reviewed By: sgatev, xazax.hun

Differential Revision: https://reviews.llvm.org/D131779