pgsql: Refactor: separate function to find all objects depending on a c

Refactor: separate function to find all objects depending on a column

Move code from ATExecAlterColumnType() that finds the all the objects
that depend on the column to a separate function.  A future patch will
reuse this code.

Author: Amul Sul <sulamul@gmail.com>
Discussion:
https://www.postgresql.org/message-id/flat/CAAJ_b94yyJeGA-5M951_Lr+KfZokOp-2kXicpmEhi5FXhBeTog@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d4e66a39eb96ca514e3f49c85cf0b4b6f138854e

Modified Files
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src/backend/commands/tablecmds.c | 408 ++++++++++++++++++++-------------------
1 file changed, 214 insertions(+), 194 deletions(-)