This PR makes the `deriving Inhabited` handler for `structure`s be able
to inherit `Inhabited` instances from structure parents, using the same
mechanism as for class parents. This fixes a regression introduced by
#9815, which lost the ability to apply `Inhabited` instances for parents
represented as subobject fields. With this PR, now it works for all
parents in the hierarchy.
Implementation detail: adds `struct_inst_default%` for synthesizing a
structure default value using `Inhabited` instances for parents and
fields.
Closes#13372
eee2909c9d