Record the proper typmod for an index expression column.
We should use exprTypmod() to extract the typmod of the expression,
instead of just blindly storing -1. This seems to have been an aboriginal
oversight in commit
fc8d970cbcdd6f025475822a4cf01dfda0873226 which
introduced general-expression indexes. The consequences are only cosmetic
at present, since the index machinery doesn't really look at typmod for
index columns; but still it seems best to describe the column type as
precisely as we can. Per off-list complaint from Thomas Fanghaenel.