partition_builder: accept_row(): use append_clustering_row()
partition_builder: accept_row(): use append_clustering_row()

The partition builder doesn't expect the looked-up row to exist. In fact
it already existing is a sign of a bug. Currently bugs resulting in
duplicate rows will manifest by tripping an assert in
`row::append_cell()`. This however results in poor diagnostics, so we
want to catch these errors sooner to be able to provide higher level
diagnostics. To this end, switch to the freshly introduced
`append_clustering_row()` so that duplicate rows are found early and in
a context where their identity is known.

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla