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.
This means it can be emitted during an -emit-module frontend job, which is the
most common place it will be used, so reusing work like this is important for
performance.
For now, this has to happen as part of a single frontend invocation, i.e. -wmo
or -force-single-frontend-invocation.