[clang][deps] Use correct DiagnosticOptions for command-line handling

In this patch the dependency scanner starts using proper `DiagnosticOptions` parsed from the actual TU command-line in order to mimic what the actual compiler would do. The actual functionality will be enabled and tested in follow-up patches. (This split is necessary to avoid temporary regression.)

Depends on D108976.

Reviewed By: dexonsmith, arphaman

Differential Revision: https://reviews.llvm.org/D108982