Change to readonly conflict handling

[Richard Kenner]

    Yes, but see: it's not the field itself that is being stored or read
    readonly, it's the whole structure.  Yet the structure is not
    readonly.

Yes, that's what readonly_fields_p is all about: to handle precisely
that case.

    I'd just decide that RTX_UNCHANGING_P is not the construct that should
    be used to represent constant fields.

Then what is?  We certainly want to indicate *somehow* that they can't
change.