Fix -Warray-bounds build error in MLAS on clang 17+ (#27499)
### Description
The `-Warray-bounds` suppression pragma in
`sqnbitgemm_kernel_avx2_int8_blklen32.h` was gated on
`defined(HAS_ARRAY_BOUNDS)`, which is set in `onnxruntime_config.h`.
MLAS never includes that header, so the guard was dead code and the
pragma never fired.
Changed the guard to `#ifdef __clang__`:
```cpp
// Before: HAS_ARRAY_BOUNDS never defined in MLAS TU
#if defined(__clang__) && defined(HAS_ARRAY_BOUNDS)
// After
#ifdef __clang__
```
Note: `__has_warning("-Warray-bounds")` was considered but the C
preprocessor does not short-circuit `&&`, so GCC fails to parse it even
behind `defined(__clang__)`.
### Motivation and Context
Build fails on Intel Mac with Apple Clang 17.0.0
(`-Werror,-Warray-bounds`). Clang raises a false-positive array-bounds
warning on `acc[4..7]` inside an `if constexpr (NCols4 == 8)` branch
that is dead when `NCols4 == 4`.
<!-- START COPILOT ORIGINAL PROMPT -->
<details>
<summary>Original prompt</summary>
>
> ----
>
> *This section details on the original issue you should resolve*
>
> <issue_title>[Build] error: array index 4 is past the end of the array
(that has type '__m256[4]') [-Werror,-Warray-bounds]</issue_title>
> <issue_description>### Describe the issue
>
> Unable to build from main branch
(0768f42400748427f1d2c39880af5aec97c8821a as of time writing this issue)
on Intel Mac
>
> ```
> /usr/bin/c++ --version
> Apple clang version 17.0.0 (clang-1700.0.13.5)
> Target: x86_64-apple-darwin24.5.0
> Thread model: posix
> InstalledDir: /Library/Developer/CommandLineTools/usr/bin
> ```
>
>
> ### Urgency
>
> _No response_
>
> ### Target platform
>
> MacOS
>
> ### Build script
>
> ./build.sh --config RelWithDebInfo --build_shared_lib --parallel
--cmake_extra_defines CMAKE_OSX_ARCHITECTURES=x86_64
>
> ### Error / output
>
> [ 18%] Building CXX object
CMakeFiles/onnxruntime_mlas.dir/onnxruntime/onnxruntime/core/mlas/lib/sqnbitgemm_kernel_avx2.cpp.o
> In file included from
/onnxruntime/onnxruntime/core/mlas/lib/sqnbitgemm_kernel_avx2.cpp:26:
>
/onnxruntime/onnxruntime/core/mlas/lib/sqnbitgemm_kernel_avx2_int8_blklen32.h:1677:49:
error: array index 4 is past the end of the array (that has type
'__m256[4]') [-Werror,-Warray-bounds]
> 1677 | __m128 acc_1 = FoldAccumulators(acc[4], acc[5], acc[6],
acc[7]);
> | ^ ~
>
/onnxruntime/onnxruntime/core/mlas/lib/sqnbitgemm_kernel_avx2_int8_blklen32.h:1531:13:
note: array 'acc' declared here
> 1531 | __m256 acc[NCols4];
> | ^
>
/onnxruntime/onnxruntime/core/mlas/lib/sqnbitgemm_kernel_avx2_int8_blklen32.h:1677:57:
error: array index 5 is past the end of the array (that has type
'__m256[4]') [-Werror,-Warray-bounds]
> 1677 | __m128 acc_1 = FoldAccumulators(acc[4], acc[5], acc[6],
acc[7]);
> | ^ ~
>
/onnxruntime/onnxruntime/core/mlas/lib/sqnbitgemm_kernel_avx2_int8_blklen32.h:1531:13:
note: array 'acc' declared here
> 1531 | __m256 acc[NCols4];
> | ^
>
/onnxruntime/onnxruntime/core/mlas/lib/sqnbitgemm_kernel_avx2_int8_blklen32.h:1677:65:
error: array index 6 is past the end of the array (that has type
'__m256[4]') [-Werror,-Warray-bounds]
> 1677 | __m128 acc_1 = FoldAccumulators(acc[4], acc[5], acc[6],
acc[7]);
> | ^ ~
>
/onnxruntime/onnxruntime/core/mlas/lib/sqnbitgemm_kernel_avx2_int8_blklen32.h:1531:13:
note: array 'acc' declared here
> 1531 | __m256 acc[NCols4];
> | ^
>
/onnxruntime/onnxruntime/core/mlas/lib/sqnbitgemm_kernel_avx2_int8_blklen32.h:1677:73:
error: array index 7 is past the end of the array (that has type
'__m256[4]') [-Werror,-Warray-bounds]
> 1677 | __m128 acc_1 = FoldAccumulators(acc[4], acc[5], acc[6],
acc[7]);
> | ^ ~
>
/onnxruntime/onnxruntime/core/mlas/lib/sqnbitgemm_kernel_avx2_int8_blklen32.h:1531:13:
note: array 'acc' declared here
> 1531 | __m256 acc[NCols4];
> | ^
> 4 errors generated.
>
> ### Visual Studio Version
>
> _No response_
>
> ### GCC / Compiler Version
>
> Apple clang version 17.0.0 (clang-1700.0.13.5)</issue_description>
>
> <agent_instructions>Please investigate the build error. If code need
fix, create a pull requests. Otherwise, suggest ways to avoid the build
errors.</agent_instructions>
>
> ## Comments on the Issue (you are @copilot in this section)
>
> <comments>
> </comments>
>
</details>
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- Fixes microsoft/onnxruntime#27497
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onnxruntime
c7c939ff - Fix -Warray-bounds build error in MLAS on clang 17+ (#27499)
