builtins: fix output of pg_typeof to be compatible

[jordanlewis]

Previously, the builtin pg_typeof was returning a CockroachDB-specific
type string for each type. This is not quite correct - we want to return
the "SQL Standard Type Name" for each datatype to match what Postgres
does.

Release note (sql change): improve compatibility of pg_typeof builtin
function.

[cockroach-teamcity]

This change is 

[blathers-crl]

❌ The GitHub CI (Cockroach) build has failed on cae8d6b8.

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[jordanlewis]

bors r+

[craig]

Build succeeded

• GitHub CI (Cockroach)