[C++][Gandiva] Add support for LLVM 23.1

[kou]

Describe the enhancement requested

It seems that LLVM 23.1.0 and 23.1.1 have been released:

https://llvm.org/

LLVM Release Schedule:

23.1.x
Tue Jul 14th, 2026: release/23.x branch
Fri Jul 17th, 2026: 23.1.0-rc1 was released
Tue Jul 28th, 2026: 23.1.0-rc2 was released
Tue Aug 12th, 2026: 23.1.0-rc3 was released
Tue Aug 25th, 2026: 23.1.0 was released
Tue Sep 8th, 2026: 23.1.1 was released
Tue Sep 22nd, 2026: 23.1.2
Tue Oct 6th, 2026: 23.1.3
Tue Oct 20th, 2026: 23.1.4
Tue Nov 3rd, 2026: 23.1.5
Tue Nov 17th, 2026: 23.1.6
Tue Dec 1st, 2026: 23.1.7
Tue Dec 15th, 2026: 23.1.8
Tue Dec 29th, 2026: 23.1.9 (if necessary)

It seems that conda provides LLVM 23.1. So our conda related CI jobs are failing:

https://github.com/ursacomputing/crossbow/actions/runs/34261627141/job/102180729366#step:7:1246

CMake Warning at cmake_modules/FindLLVMAlt.cmake:30 (find_package):
  Could not find a configuration file for package "LLVM" that is compatible
  with requested version "22.1".

  The following configuration files were considered but not accepted:

    /opt/conda/envs/arrow/lib/cmake/llvm/LLVMConfig.cmake, version: 23.1.0
      The version found is not compatible with the version requested.

Call Stack (most recent call first):
  src/gandiva/CMakeLists.txt:39 (find_package)

Component(s)

C++, Gandiva

[raulcd]

This is also currently failing on my PR on the test-conda-cpp job:
https://github.com/ursacomputing/crossbow/actions/runs/34324045693/job/102377095609

[HuaHuaY]

• Since LLVM 23 added -Wunused-template to -Wall, some code requires modification. It also exposed an issue in CastImpl about Date and Timestamp data type. https://releases.llvm.org/23.1.0/tools/clang/docs/ReleaseNotes.html

  -Wunused-template is now part of -Wunused (which is enabled by -Wall). It diagnoses unused function and variable templates with internal linkage, which in a header is a latent ODR hazard. It can be disabled with -Wno-unused-template. (#202945)

• I am investigating a tricky issue in cpp/src/gandiva/llvm_generator_test.cc. Under macOS debug build, LLVM 23 is no longer generating vectorized code, causing the TEST_F(TestLLVMGenerator, TestAdd) test to fail. https://releases.llvm.org/23.1.0/docs/ReleaseNotes.html#changes-to-the-llvm-ir

  alwaysinline no longer bypasses inlining compatibility checks based on target features. Inlining will only be performed if it is safe to do so.