Always set `safeInferred`, not only when it turns `False`

previously, safeFlagCheck would be happy to switch the safeFlag to False, but not the other way around. This meant that after

:set -XGeneralizedNewtypeDeriving
:set -XNoGeneralizedNewtypeDeriving

in GHCi all loaded files would be still be infered as unsafe.

This fixes #19243 (closed).

This is a corner case, but somewhat relevant once ghci by default starts with GeneralizedNewtypeDeriving on (due to GHC2021).