Fix CLUSTER on expression indexes.

Fix CLUSTER on expression indexes.

Since the introduction of different slot types, in 1a0586de3657, we
create a virtual slot in tuplesort_begin_cluster(). While that looks
right, it unfortunately doesn't actually work, as ExecStoreHeapTuple()
is used to store tuples in the slot. Unfortunately no regression tests
for CLUSTER on expression indexes existed so far.

Fix the slot type, and add bare bones tests for CLUSTER on expression
indexes.

Reported-By: Justin Pryzby
Author: Andres Freund
Discussion: https://postgr.es/m/20191011210320.GS10470@telsasoft.com
Backpatch: 12, like 1a0586de3657