Amit Langote reported that partition prune was unable to work with
arrays, enums, etc, which led him to research the appropriate way to
match query clauses to partition keys: instead of searching for an exact
match of the expression's type, it is better to rely on the fact that
the expression qual has already been resolved to a specific operator,
and that the partition key is linked to a specific operator family.
With that info, it's possible to figure out the strategy and comparison
function to use for the pruning clause in a manner that works reliably
for pseudo-types also.

Include new test cases that demonstrate pruning where pseudotypes are
involved.

Author: Amit Langote, Álvaro Herrera
Discussion: https://postgr.es/m/2b02f1e9-9812-9c41-972d-517bdc0f815d@lab.ntt.co.jp