github: Add setup for running libcxx's tests on the built toolchain

Keep this disabled by default (since it takes a fair amount of time
and has good test coverage upstream), but including it in disabled
form so that it's easy to enable temporarily for testing occasionally.

Libcxx does test the mingw configuration in their own CI (with an older
pinned release of the compiler), so this is mostly redundant, and
primarily relevant for testing that the very latest compiler (and
python setup) works for the tests.

.github/workflows/build.yml

@@ -268,6 +268,72 @@ jobs:
           ./run-tests.sh /llvm-mingw
           ./run-lldb-tests.sh /llvm-mingw
 
+  # Run libcxx's tests with the cross-built i686/x86_64 toolchains from above.
+  # (This builds its own copy of libcxx, but it should be pretty much
+  # identical to the one shipped - and tests that the toolchain works
+  # for running the libcxx tests.)
+  # This also forces testing the bundled python executables.
+  test-libcxx:
+    # This test is kinda slow, and kinda redundant (libcxx does contain
+    # llvm-mingw configurations in its own CI, so the only difference with
+    # testing here is different runners with different OS setups, and
+    # testing with the latest compiler instead of an older release).
+    # Therefore, keep the test disabled by default; it's easy to enable
+    # temporarily on a branch for testing.
+    if: false
+    needs: [linux-cross]
+    runs-on: windows-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        arch:
+          - x86_64
+          - i686
+    steps:
+      - uses: actions/download-artifact@v3
+        with:
+          name: windows-${{matrix.arch}}-toolchain
+      - name: Unpack toolchain
+        run: |
+          Expand-Archive llvm-mingw-*.zip -DestinationPath .
+          del llvm-mingw-*.zip
+          mv llvm-mingw-* c:\llvm-mingw
+          echo "c:\llvm-mingw\bin" | Out-File -FilePath $Env:GITHUB_PATH -Encoding utf8 -Append
+          echo "c:\llvm-mingw\python\bin" | Out-File -FilePath $Env:GITHUB_PATH -Encoding utf8 -Append
+          echo "PYTHON_EXE=c:/llvm-mingw/python/bin/python3.exe" | Out-File -FilePath $Env:GITHUB_ENV -Encoding utf8 -Append
+      - name: Fetch version
+        if: github.event_name == 'schedule'
+        run: |
+          cat c:\llvm-mingw\versions | Out-File -FilePath $Env:GITHUB_ENV -Encoding utf8 -Append
+      - uses: actions/checkout@v3
+      - name: Checkout llvm-project
+        run: |
+          bash -c "CHECKOUT_ONLY=1 ./build-llvm.sh"
+      - name: Build and test libcxx
+        run: |
+          cd llvm-project
+          mkdir build
+          cd build
+          cmake ../runtimes `
+            -G Ninja `
+            -DCMAKE_BUILD_TYPE=Release `
+            -DPython3_EXECUTABLE="$Env:PYTHON_EXE" `
+            -DLIBCXX_ENABLE_WERROR=YES `
+            -DLLVM_ENABLE_RUNTIMES="libcxx;libcxxabi;libunwind" `
+            -DLIBCXX_CXX_ABI=libcxxabi `
+            -DCMAKE_C_COMPILER=clang `
+            -DCMAKE_CXX_COMPILER=clang++ `
+            -DLIBCXX_TARGET_INFO="libcxx.test.target_info.MingwLocalTI" `
+            -DLIBCXXABI_ENABLE_SHARED=NO `
+            -DLIBCXX_ENABLE_STATIC_ABI_LIBRARY=YES `
+            -DLIBCXX_USE_COMPILER_RT=YES `
+            -DLIBCXXABI_USE_COMPILER_RT=YES `
+            -DLIBUNWIND_USE_COMPILER_RT=YES `
+            -DLIBCXXABI_USE_LLVM_UNWINDER=YES `
+            -DLIBCXX_EXTRA_SITE_DEFINES="__USE_MINGW_ANSI_STDIO=1;TEST_IS_EXECUTED_IN_A_SLOW_ENVIRONMENT"
+          ninja
+          ninja check-cxx
+
   # Run the OpenMP tests with the cross-built i686/x86_64 toolchains from above.
   # This also forces testing the bundled python executables.
   test-openmp: