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ARROW-16280: [C++] Avoid copying shared_ptr in Expression::type()

I see big improvements in the `ExecuteScalarExpressionOverhead` benchmarks, especially `ref_only_expression` with this, for example:
```
before:
ExecuteScalarExpressionOverhead/ref_only_expression/rows_per_batch:1000000/real_time/threads:1         35.4 ns         35.4 ns     19577007 batches_per_second=28.2395M/s rows_per_second=28.2395T/s
ExecuteScalarExpressionOverhead/ref_only_expression/rows_per_batch:1000000/real_time/threads:16        49.8 ns          788 ns     14280992 batches_per_second=20.0734M/s rows_per_second=20.0734T/s

after:
ExecuteScalarExpressionOverhead/ref_only_expression/rows_per_batch:1000000/real_time/threads:1         27.6 ns         27.5 ns     25090317 batches_per_second=36.2832M/s rows_per_second=36.2832T/s
ExecuteScalarExpressionOverhead/ref_only_expression/rows_per_batch:1000000/real_time/threads:16        4.26 ns         67.2 ns    184745728 batches_per_second=235.006M/s rows_per_second=235.006T/s

```

Also the overhead of small batch size/multithreaded benchmarks is reduced, for example in `complex_expression`:
```
before:
ExecuteScalarExpressionOverhead/complex_expression/rows_per_batch:1000/real_time/threads:1          3723682 ns      3721326 ns          191 batches_per_second=268.551k/s rows_per_second=268.551M/s
ExecuteScalarExpressionOverhead/complex_expression/rows_per_batch:1000/real_time/threads:16         1153070 ns     18365265 ns          624 batches_per_second=867.25k/s rows_per_second=867.25M/s

after:
ExecuteScalarExpressionOverhead/complex_expression/rows_per_batch:1000/real_time/threads:1          3543745 ns      3541909 ns          197 batches_per_second=282.187k/s rows_per_second=282.187M/s
ExecuteScalarExpressionOverhead/complex_expression/rows_per_batch:1000/real_time/threads:16          841776 ns     13395266 ns          944 batches_per_second=1.18796M/s rows_per_second=1.18796G/s
```

Closes #12957 from zagto/datatype-performace-expression-type

Authored-by: Tobias Zagorni <tobias@zagorni.eu>
Signed-off-by: David Li <li.davidm96@gmail.com>