Refactor construct_array() and deconstruct_array() for built-in types

[for PG16]

There are many calls to construct_array() and deconstruct_array() for 
built-in types, for example, when dealing with system catalog columns. 
These all hardcode the type attributes necessary to pass to these functions.

To simplify this a bit, add construct_array_builtin(), 
deconstruct_array_builtin() as wrappers that centralize this hardcoded 
knowledge.  This simplifies many call sites and reduces the amount of 
hardcoded stuff that is spread around.

I also considered having genbki.pl generate lookup tables for these 
hardcoded values, similar to schemapg.h, but that ultimately seemed 
excessive.

Thoughts?