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.

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