[clang] Add a Windows build in the Clang pre-commit CI

[ldionne]

Summary

This patch adds a CI job for Clang on Windows that is separate from
the monolithic job that gets added automatically via the Phabricator
integration with Buildkite. This way, we will retain the Windows testing
for Clang when we move to GitHub Pull Requests.

[ldionne]

@aaron.ballman for awareness

[aaron.ballman]

In D158995#4621441, @ldionne wrote:

@aaron.ballman for awareness

Thank you for this!

[ldionne]

Run script through bash on Windows.

[ldionne]

Poke CI

[ldionne]

Set VC variables

[ldionne]

Poke CI -- it seemed to time out this afternoon due to Phab.

[ldionne]

Don't upload the Windows build artifacts cause we don't use them and it fails.

[ldionne]

Poke CI -- Phabricator keeps timing out.

[ldionne]

This looks good, the Windows CI is failing with the original premerge checks in exactly the same way as this new job, so I guess it's a current issue on main.

clang/utils/ci/buildkite-pipeline.yml

- label: "Format"
     retry:
       automatic:
         - exit_status: -1  # Agent was lost
           limit: 2
     timeout_in_minutes: 120
 
   - wait
 
-  - label: "Building and testing clang"
+  - label: "Building and testing clang (Linux)"
     commands:
       - "clang/utils/ci/run-buildbot build-clang"
     agents:
       queue: "linux"
     retry:
       automatic:
         - exit_status: -1  # Agent was lost
           limit: 2
     timeout_in_minutes: 120
 
+  - label: "Building and testing clang (Windows)"
+    commands:
+      - "bash clang/utils/ci/run-buildbot build-clang-windows"
+    agents:
+      queue: "windows"
+    retry:
+      automatic:
+        - exit_status: -1  # Agent was lost
+          limit: 2
+    timeout_in_minutes: 120
+
   - wait
 
   - label: "Running libc++ test suite in C++03"
     commands:
       - "clang/utils/ci/run-buildbot generic-cxx03"
     artifact_paths:
       - "**/test-results.xml"
       - "**/crash_diagnostics/*"

clang/utils/ci/run-buildbot

build-clang)
 
     ninja -C ${BUILD_DIR} install-clang install-clang-resource-headers
     ccache -s
     tar -cJvf install.tar.xz install/
     buildkite-agent artifact upload --debug install.tar.xz
 
     ninja -C ${BUILD_DIR} check-clang
 ;;
+build-clang-windows)
+    cmake -S llvm -B ${BUILD_DIR} -G Ninja                                      \
+        -D CMAKE_C_COMPILER_LAUNCHER=sccache                                    \
+        -D CMAKE_CXX_COMPILER_LAUNCHER=sccache                                  \
+        -D CMAKE_BUILD_TYPE=Release                                             \
+        -D CMAKE_INSTALL_PREFIX=install-windows                                 \
+        -D LLVM_ENABLE_PROJECTS="clang;compiler-rt"                             \
+        -D LLVM_ENABLE_ASSERTIONS=ON                                            \
+        -D LLVM_BUILD_EXAMPLES=ON                                               \
+        -D COMPILER_RT_BUILD_LIBFUZZER=OFF                                      \
+        -D COMPILER_RT_BUILD_ORC=OFF
+
+    ninja -C ${BUILD_DIR} install-clang install-clang-resource-headers
+    tar -cJvf install-windows.tar.xz install-windows/
+    buildkite-agent artifact upload --debug install-windows.tar.xz
+
+    ninja -C ${BUILD_DIR} check-clang
+;;
 generic-cxx03)
     buildkite-agent artifact download install.tar.xz .
     tar -xvf install.tar.xz
     export CC=$(pwd)/install/bin/clang
     export CXX=$(pwd)/install/bin/clang++
     chmod +x install/bin/clang install/bin/clang++
 
     clean