pgsql: Refactor assembly of external on-disk TOAST pointers

Refactor assembly of external on-disk TOAST pointers

toast_save_datum() ended with two copies of the same three steps, one
each external on-disk TOAST pointer: allocate a datum, set its tag, then
copy the contents from a varatt_external_oid or a varatt_external_oid8
into it.

These steps are moved into a new helper function, toast_pointer_build(),
which takes the tag and a pointer to the struct and derives everything
else from the tag. For now, only two varatt_external_* exist for
external on-disk TOAST pointers. Some more are under discussion, where
this refactoring shows some benefits in the shape of less duplication.

There should be no behavior change.

Author: Nikhil Kumar Veldanda <veldanda(dot)nikhilkumar17(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAFAfj_FeJQZrKyVn7ur0aJx8iz8SR14NheVJNyc8JNoqtMX+og@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/74f7f684d6ed1fd3c3aee4d868f2e6d1da0ec93b

Modified Files
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src/backend/access/common/toast_internals.c | 28 ++++++++++++++++++++++------
1 file changed, 22 insertions(+), 6 deletions(-)