CI: Use LLVM's OpenMP over MSVC_DEBUG_non_redist on Windows (#26678)

* CI: Use LLVM's OpenMP over MSFT_DEBUG_non_redist on Windows

Currently, we ship the non-redist debug version of microsoft's libomp.
This PR changes this to official LLVM's release, also packaging
the license as needed.

* Remove LLVM SHA from job name to increase legibility

* Add temp validations to CI

* Revert "Add temp validations to CI"

This reverts commit eef97c88b5bac280803ebb3c3b7bb09f89b0fd88.

* Build OpenMP in CI

* Make OpenMP fetch self-contained in cmake and cache in CI

* Robustify Licens-packaging

1. Ship OpenMP license, not LLVM's.
2. Invalidate cache also on checksum of the license

* Remove stale reference in docs/build.md

* No longer package base license in release

This was scope-creep

* Add explanatory comment to OpenMP license

* Remove arm64 smoke

Forgot this during conflict resolution during rebase of
c54c0e9cf6030a5a54ce8bdd81b3e146d9787d42

* Remove GGML_OPENMP_FETCH_CACHE_DIR as requested by @CISC

* whitespace changes

.github/workflows/build-cpu.yml

@@ -119,6 +119,7 @@ jobs:
           ./bin/llama-completion -m stories260K.gguf -p "One day, Lily met a Shoggoth" -n 500 -c 256
 
   windows:
+    name: windows / ${{ matrix.build }}
     runs-on: windows-2025
 
     env:
@@ -130,13 +131,13 @@ jobs:
         include:
           - build: 'x64-cpu-static'
             arch: 'x64'
-            defines: '-G "Ninja Multi-Config" -D CMAKE_TOOLCHAIN_FILE=cmake/x64-windows-llvm.cmake -DGGML_NATIVE=OFF -DLLAMA_BUILD_SERVER=ON -DGGML_RPC=ON -DBUILD_SHARED_LIBS=OFF'
+            defines: '-G "Ninja Multi-Config" -D CMAKE_TOOLCHAIN_FILE=cmake/x64-windows-llvm.cmake -DGGML_NATIVE=OFF -DGGML_OPENMP_FETCH=ON -DLLAMA_BUILD_SERVER=ON -DGGML_RPC=ON -DBUILD_SHARED_LIBS=OFF'
           - build: 'x64-openblas'
             arch: 'x64'
             defines: '-G "Ninja Multi-Config" -D CMAKE_TOOLCHAIN_FILE=cmake/x64-windows-llvm.cmake -DGGML_NATIVE=OFF -DLLAMA_BUILD_SERVER=ON -DGGML_RPC=ON -DGGML_BACKEND_DL=ON -DGGML_CPU_ALL_VARIANTS=ON -DGGML_OPENMP=OFF -DGGML_BLAS=ON -DGGML_BLAS_VENDOR=OpenBLAS -DBLAS_INCLUDE_DIRS="$env:RUNNER_TEMP/openblas/include" -DBLAS_LIBRARIES="$env:RUNNER_TEMP/openblas/lib/openblas.lib"'
           - build: 'arm64'
             arch: 'arm64'
-            defines: '-G "Ninja Multi-Config" -D CMAKE_TOOLCHAIN_FILE=cmake/arm64-windows-llvm.cmake -DGGML_NATIVE=OFF -DLLAMA_BUILD_SERVER=ON'
+            defines: '-G "Ninja Multi-Config" -D CMAKE_TOOLCHAIN_FILE=cmake/arm64-windows-llvm.cmake -DGGML_NATIVE=OFF -DGGML_OPENMP_FETCH=ON -DLLAMA_BUILD_SERVER=ON'
 
     steps:
       - name: Clone

.github/workflows/release.yml

@@ -681,6 +681,7 @@ jobs:
           name: llama-bin-win-openvino-${{ env.OPENVINO_VERSION_MAJOR }}-x64.zip
 
   windows-cpu:
+    name: windows-cpu / ${{ matrix.arch }}
     needs: [check-release]
     if: ${{ needs.check-release.outputs.should_release == 'true' }}
 
@@ -728,6 +729,7 @@ jobs:
             -DGGML_BACKEND_DL=ON ^
             -DGGML_CPU_ALL_VARIANTS=${{ matrix.arch == 'x64' && 'ON' || 'OFF' }} ^
             -DGGML_OPENMP=ON ^
+            -DGGML_OPENMP_FETCH=ON ^
             ${{ env.CMAKE_ARGS }}
           cmake --build build --config Release
 
@@ -739,7 +741,6 @@ jobs:
       - name: Pack artifacts
         id: pack_artifacts
         run: |
-          Copy-Item "C:\Program Files\Microsoft Visual Studio\18\Enterprise\VC\Redist\MSVC\14.51.36231\debug_nonredist\${{ matrix.arch }}\Microsoft.VC145.OpenMP.LLVM\libomp140.${{ matrix.arch == 'x64' && 'x86_64' || 'aarch64' }}.dll" .\build\bin\Release\
           7z a -snl llama-bin-win-cpu-${{ matrix.arch }}.zip .\build\bin\Release\*
 
       - name: Upload artifacts