Always keep unset fields in FPOptionsOverride zeroed.

There are three fields that the FPOptions default constructor sets to
non-zero values; those fields previously could have been zero or
non-zero depending on whether they'd been explicitly removed from the
FPOptionsOverride set. However, that doesn't seem to ever actually
happen, so this is NFC, except that it makes the AST file representation
of FPOptionsOverride make more sense.