iface: Store flags in interface files

When reporting the reason why a module is recompiled (using
`-dump-hi-diffs`), it is much more informative to inform the user about
which flag exactly has changed, rather than just an opaque reference to
a hash.

Now, when the user enables `-fwrite-if-self-recomp-flags`
there is a difference the precise part of the flags is
reported:

```
codegen flags changed:
before: [Opt_NoTypeableBinds, Opt_OmitYields]
after: [Opt_NoTypeableBinds, Opt_OmitYields, Opt_DictsStrict]
```

Fixes #25571