CREATE TABLE LIKE INCLUDING GENERATED would fail if a generated column
referred to a column with a higher attribute number.  This is because
the column mapping mechanism created the mapping incrementally as
columns are added.  This was sufficient for previous uses of that
mechanism (omitting dropped columns), and it also happened to work if
generated columns only referred to columns with lower attribute
numbers, but here it failed.

This fix is to build the attribute mapping in a separate loop before
processing the columns in detail.

Bug: #16342
Reported-by: Ethan Waldo <ewaldo@healthetechs.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>