Frontend options should only be printed under `swift-module-flags-ignored:`
temporarily to prevent condfails for older development compilers that are still
in use (as a rule of thumb, swiftinterfaces should be compatible with compilers
that are up to 6 months old). None of the frontend flags that are currently
categorized as "ignorable" need to be ignorable anymore.
- Add driver and frontend option
- Add LangOptions entry
- Ensure driver propagates flag to frontends
- Add feature to `features.json`
Part of rdar://91119995
The `-force-single-frontend-invocation` flag predates WMO and is now an
alias for `-whole-module-optimization`. We should use the latter and let
the former fade into history.
