ARROW-16205: [C++][FlightRPC] Don't use constexpr std::initializer_list

[lidavidm]

To fix a build error on Clang.

[github-actions]

https://issues.apache.org/jira/browse/ARROW-16205

[github-actions]

⚠️ Ticket has no components in JIRA, make sure you assign one.

[lidavidm]

@github-actions crossbow submit verify-rc-source-cpp-macos-*

[github-actions]

Revision: 2dc7c3c

Submitted crossbow builds: ursacomputing/crossbow @ actions-1866

Task	Status
verify-rc-source-cpp-macos-amd64
verify-rc-source-cpp-macos-arm64
verify-rc-source-cpp-macos-conda-amd64

[kou]

+1

Thanks!

[ursabot]

Benchmark runs are scheduled for baseline = d36b2b3 and contender = 83dda5c. 83dda5c is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Finished ⬇️0.0% ⬆️0.0%] ec2-t3-xlarge-us-east-2
[Failed ⬇️0.38% ⬆️0.13%] test-mac-arm
[Failed ⬇️0.36% ⬆️0.0%] ursa-i9-9960x
[Finished ⬇️0.21% ⬆️0.0%] ursa-thinkcentre-m75q
Buildkite builds:
[Finished] <https://buildkite.com/apache-arrow/arrow-bci-benchmark-on-ec2-t3-xlarge-us-east-2/builds/523|83dda5c3 ec2-t3-xlarge-us-east-2>
[Failed] <https://buildkite.com/apache-arrow/arrow-bci-benchmark-on-test-mac-arm/builds/510|83dda5c3 test-mac-arm>
[Failed] <https://buildkite.com/apache-arrow/arrow-bci-benchmark-on-ursa-i9-9960x/builds/509|83dda5c3 ursa-i9-9960x>
[Finished] <https://buildkite.com/apache-arrow/arrow-bci-benchmark-on-ursa-thinkcentre-m75q/builds/520|83dda5c3 ursa-thinkcentre-m75q>
[Finished] <https://buildkite.com/apache-arrow/arrow-bci-benchmark-on-ec2-t3-xlarge-us-east-2/builds/522|d36b2b33 ec2-t3-xlarge-us-east-2>
[Finished] <https://buildkite.com/apache-arrow/arrow-bci-benchmark-on-test-mac-arm/builds/509|d36b2b33 test-mac-arm>
[Finished] <https://buildkite.com/apache-arrow/arrow-bci-benchmark-on-ursa-i9-9960x/builds/508|d36b2b33 ursa-i9-9960x>
[Finished] <https://buildkite.com/apache-arrow/arrow-bci-benchmark-on-ursa-thinkcentre-m75q/builds/519|d36b2b33 ursa-thinkcentre-m75q>
Supported benchmarks:
ec2-t3-xlarge-us-east-2: Supported benchmark langs: Python, R. Runs only benchmarks with cloud = True
test-mac-arm: Supported benchmark langs: C++, Python, R
ursa-i9-9960x: Supported benchmark langs: Python, R, JavaScript
ursa-thinkcentre-m75q: Supported benchmark langs: C++, Java