GH-50636: [C++][R] Fix macOS 11.3 std::span construction in vector_sort

[KHARSHAVARDHAN-eng]

Rationale for this change

Building Apache Arrow on macOS using older LLVM libc++ toolchains (such as Xcode 12.5 shipped with the macOS 11.3 SDK) fails when compiling the vector_sort compute kernel due to unsupported std::span range constructors.

What changes are included in this PR?

In cpp/src/arrow/compute/kernels/vector_sort_internal.h:

• In NonStablePartitioner & StablePartitioner: Replaced non-conforming std::span<uint64_t>{middle, indices.data() + indices.size()} with indices.subspan(middle - indices.data()).
• In ChunkedMergeImpl::MergeNullsAtStart & MergeNullsAtEnd: Replaced std::span<CompressedChunkLocation>{left.overall_begin(), right.overall_end()} with std::span<CompressedChunkLocation>{left.overall_begin(), static_cast<size_t>(right.overall_end() - left.overall_begin())}.

Both .subspan(offset) and std::span(pointer, size_type) are standard C++20 and supported on older macOS libc++ implementations.

Verification

• Built affected compute targets successfully.
• Built vector sort tests successfully.
• Verified the diff only contains the intended changes.

Closes #50636

• GitHub Issue: [C++][R] macOS 11.3 SDK fails to compile std::span iterator range in vector_sort #50636

[github-actions]

⚠️ GitHub issue #50636 has been automatically assigned in GitHub to PR creator.

[kou]

@github-actions crossbow submit -g r

[kou]

Could you use our PR template? (You can make PR ready after it.)

Could you read https://arrow.apache.org/docs/dev/developers/overview.html#ai-generated-code ?

[github-actions]

Revision: 99e8db3

Submitted crossbow builds: ursacomputing/crossbow @ actions-5c2ee98607

Task	Status
r-binary-packages
r-recheck-most
test-r-alpine-linux-cran
test-r-arrow-backwards-compatibility
test-r-depsource-system
test-r-dev-duckdb
test-r-devdocs
test-r-extra-packages
test-r-fedora-clang
test-r-gcc-11
test-r-gcc-12
test-r-install-local
test-r-install-local-minsizerel
test-r-linux-as-cran
test-r-linux-rchk
test-r-linux-sanitizers
test-r-linux-valgrind
test-r-m1-san
test-r-macos-as-cran
test-r-offline-maximal
test-r-ubuntu-22.04
test-r-versions
test-r-wasm

[Reranko05]

The issue description points to the std::span construction in vector_sort.cc, but this PR only changes vector_sort_internal.h. Also, the test-r-macos-as-cran Crossbow job is still failing with the same cannot convert initializer list argument to std::span<uint64_t> error in vector_sort.cc. Could you clarify how these changes resolve the compilation failure reported in GH-50636?