[C++] Support scalar columns in hash aggregations (was: hash_sum on scalar column segfaults)

[asfimport]

Reproducer in R. Works fine without grouping, but segfaults if you group:

tab <- Table$create(a=1:4, b=6:9)

tab %>% 
 mutate(counter = 1) %>% 
 summarize(count = sum(counter))

Table
1 rows x 1 columns
$count <double>

tab %>% 
 mutate(counter = 1) %>% 
 group_by(a) %>% 
 summarize(count = sum(counter))

 *** caught segfault ***
address 0x0, cause 'invalid permissions'

Traceback:
 1: .Call(`_arrow_ExecPlan_run`, plan, final_node)
 2: ExecPlan_run(self, node)
 3: plan$Run(final_node)
 4: do_exec_plan(.data, group_vars = dplyr::group_vars(.data))
 5: do_arrow_summarize(.data, ...)
...

Reporter: Neal Richardson / @nealrichardson
Assignee: David Li / @lidavidm

Related issues:

• [R] Bindings for count aggregation (blocks)

PRs and other links:

• GitHub Pull Request #10994

_{Note: This issue was originally created as ARROW-13737. Please see the migration documentation for further details.}

[asfimport]

Neal Richardson / @nealrichardson:
Issue resolved by pull request 10994
#10994