[llvm] [Support] System include SipHash.h (PR #149499)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-arm64-windows-msvc` running on `linaro-armv8-windows-msvc-04` while building `llvm` at step 7 "test-build-unified-tree-check-runtimes".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/161/builds/7099

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 7 (test-build-unified-tree-check-runtimes) failure: test (failure)
******************** TEST 'libomp :: tasking/issue-94260-2.c' FAILED ********************
Exit Code: 3221225477

Command Output (stdout):
--
# RUN: at line 1
C:/Users/tcwg/llvm-worker/clang-arm64-windows-msvc/build/./bin/clang.exe -fopenmp   -I C:/Users/tcwg/llvm-worker/clang-arm64-windows-msvc/build/runtimes/runtimes-bins/openmp/runtime/src -I C:/Users/tcwg/llvm-worker/clang-arm64-windows-msvc/llvm-project/openmp/runtime/test -L C:/Users/tcwg/llvm-worker/clang-arm64-windows-msvc/build/runtimes/runtimes-bins/openmp/runtime/src  -fno-omit-frame-pointer C:\Users\tcwg\llvm-worker\clang-arm64-windows-msvc\llvm-project\openmp\runtime\test\tasking\issue-94260-2.c -o C:\Users\tcwg\llvm-worker\clang-arm64-windows-msvc\build\runtimes\runtimes-bins\openmp\runtime\test\tasking\Output\issue-94260-2.c.tmp && C:\Users\tcwg\llvm-worker\clang-arm64-windows-msvc\build\runtimes\runtimes-bins\openmp\runtime\test\tasking\Output\issue-94260-2.c.tmp
# executed command: C:/Users/tcwg/llvm-worker/clang-arm64-windows-msvc/build/./bin/clang.exe -fopenmp -I C:/Users/tcwg/llvm-worker/clang-arm64-windows-msvc/build/runtimes/runtimes-bins/openmp/runtime/src -I C:/Users/tcwg/llvm-worker/clang-arm64-windows-msvc/llvm-project/openmp/runtime/test -L C:/Users/tcwg/llvm-worker/clang-arm64-windows-msvc/build/runtimes/runtimes-bins/openmp/runtime/src -fno-omit-frame-pointer 'C:\Users\tcwg\llvm-worker\clang-arm64-windows-msvc\llvm-project\openmp\runtime\test\tasking\issue-94260-2.c' -o 'C:\Users\tcwg\llvm-worker\clang-arm64-windows-msvc\build\runtimes\runtimes-bins\openmp\runtime\test\tasking\Output\issue-94260-2.c.tmp'
# executed command: 'C:\Users\tcwg\llvm-worker\clang-arm64-windows-msvc\build\runtimes\runtimes-bins\openmp\runtime\test\tasking\Output\issue-94260-2.c.tmp'
# note: command had no output on stdout or stderr
# error: command failed with exit status: 0xc0000005

--

********************


```

</details>

https://github.com/llvm/llvm-project/pull/149499