Fix CREATE TABLE LIKE INCLUDING GENERATED column order issue

 Fix CREATE TABLE LIKE INCLUDING GENERATED column order issue
 
 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>