Allow virtual columns in index expressions or predicate

I'm looking for a solution to support virtual generated columns in
index expressions or predicate. The comment in DefineIndex() seems to
suggest that virtual generated columns there can be expanded in
RelationGetIndexExpressions() and RelationGetIndexPredicate().

/*
* XXX Virtual generated columns in index expressions or predicates
* could be supported, but it needs support in
* RelationGetIndexExpressions() and RelationGetIndexPredicate().
*/

I'm not sure this is the appropriate approach, especially since we
don't have the heap relation available in these functions. I think
we're good as long as we expand the virtual columns before the index
build process; otherwise, evaluating these expressions during the
build will cause executor errors due to unexpanded virtual columns.

I wonder if we can do the expansion in DefineIndex(), before the call
to index_create(), like the attached.

- Richard