Make cast function from circle to polygon error safe

Make cast function from circle to polygon error safe

Previously, the function casting type circle to type polygon could not
be made error safe, because it is an SQL language function.

This refactors it as a C/internal function, by sharing code with the
C/internal function that the SQL function previously wrapped, and soft
error support is added.

Author: jian he <jian.universality@gmail.com>
Reviewed-by: Amul Sul <sulamul@gmail.com>
Reviewed-by: Corey Huinker <corey.huinker@gmail.com>
Discussion: Discussion: https://www.postgresql.org/message-id/flat/CADkLM%3Dfv1JfY4Ufa-jcwwNbjQixNViskQ8jZu3Tz_p656i_4hQ%40mail.gmail.com