Need to special-case RECORD as well as UNKNOWN in plpgsql's casting logic.

Need to special-case RECORD as well as UNKNOWN in plpgsql's casting logic.

This is because can_coerce_type thinks that RECORD can be cast to any
composite type, but coerce_record_to_complex only works for inputs that are
RowExprs or whole-row Vars, so we get a hard failure on a CaseTestExpr.

Perhaps these corner cases ought to be fixed so that coerce_to_target_type
actually returns NULL as per its specification, rather than failing ...
but for the moment an extra check here is the path of least resistance.