Record the proper typmod for an index expression column.

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.