driver: fix foreign stub handling logic in hscParsedDecls

This patch fixes foreign stub handling logic in `hscParsedDecls`.
Previously foreign stubs were simply ignored here, so any feature that
involve foreign stubs would not work in ghci (e.g. CApiFFI). The patch
reuses `generateByteCode` logic and eliminates a large chunk of
duplicate logic that implements Core to bytecode generation pipeline
here. Fixes #25414.