[libcxx] [ci] Add a i386 Windows configuration to CI

[mstorsjo]

Summary

Adding a mingw based config is easy in the current CI environment
(where we can just choose the different target by calling
i686-w64-mingw32-clang), while adding a clang-cl based config would
require setting up different environment variables pointing to the
i386 library directory.

Just adding one config (DLL) instead of exhaustively testing both
(DLL and static) as very few tests would differ in practice, to keep
the CI load reasonable.

A couple tests still fail in this config - they are fixed by D124989
and D124990.

[ldionne]

LGTM with green CI

[mstorsjo]

Rebased after landing fixes for the tests; the new CI config should now pass.

libcxx/docs/index.rst

 Libc++ also supports common platforms and architectures:
 
 =============== ========================= ============================
 Target platform Target architecture       Notes
 =============== ========================= ============================
 macOS 10.9+     i386, x86_64, arm64       Building the shared library itself requires targetting macOS 10.11+
 FreeBSD 10+     i386, x86_64, arm
 Linux           i386, x86_64, arm, arm64
-Windows         x86_64                    Both MSVC and MinGW style environments
+Windows         i386, x86_64              Both MSVC and MinGW style environments
 AIX             powerpc, powerpc64
 =============== ========================= ============================
 
 Generally speaking, libc++ should work on any platform that provides a fairly complete
 C Standard Library. It is also possible to turn off parts of the library for use on
 systems that provide incomplete support.
 
 However, libc++ aims to provide a high-quality implementation of the C++ Standard

libcxx/utils/ci/buildkite-pipeline.yml

- label: "Clang-cl (Static)"
       agents:
         queue: "windows"
       retry:
         automatic:
           - exit_status: -1  # Agent was lost
             limit: 2
       timeout_in_minutes: 120
 
-    - label: "MinGW (DLL)"
+    - label: "MinGW (DLL, x86_64)"
       command: "bash libcxx/utils/ci/run-buildbot mingw-dll"
       artifact_paths:
         - "**/test-results.xml"
         - "**/*.abilist"
       agents:
         queue: "windows"
       retry:
         automatic:
           - exit_status: -1  # Agent was lost
             limit: 2
       timeout_in_minutes: 120
 
-    - label: "MinGW (Static)"
+    - label: "MinGW (Static, x86_64)"
       command: "bash libcxx/utils/ci/run-buildbot mingw-static"
       artifact_paths:
         - "**/test-results.xml"
         - "**/*.abilist"
       agents:
         queue: "windows"
       retry:
         automatic:
           - exit_status: -1  # Agent was lost
             limit: 2
       timeout_in_minutes: 120
 
+    - label: "MinGW (DLL, i686)"
+      command: "bash libcxx/utils/ci/run-buildbot mingw-dll-i686"
+      artifact_paths:
+        - "**/test-results.xml"
+        - "**/*.abilist"
+      agents:
+        queue: "windows"
+      retry:
+        automatic:
+          - exit_status: -1  # Agent was lost
+            limit: 2
+      timeout_in_minutes: 120
+
   - group: ":apple: Apple"
     steps:
     - label: "MacOS x86_64"
       command: "libcxx/utils/ci/run-buildbot generic-cxx20"
       artifact_paths:
         - "**/test-results.xml"
         - "**/*.abilist"
       agents:

libcxx/utils/ci/run-buildbot

generate-cmake \
           -DCMAKE_C_COMPILER=x86_64-w64-mingw32-clang \
           -DCMAKE_CXX_COMPILER=x86_64-w64-mingw32-clang++ \
           -C "${MONOREPO_ROOT}/libcxx/cmake/caches/MinGW.cmake" \
           -DLIBCXX_ENABLE_SHARED=OFF \
           -DLIBUNWIND_ENABLE_SHARED=OFF
     echo "+++ Running the libc++ tests"
     ${NINJA} -vC "${BUILD_DIR}" check-cxx
 ;;
+mingw-dll-i686)
+    clean
+    generate-cmake \
+          -DCMAKE_C_COMPILER=i686-w64-mingw32-clang \
+          -DCMAKE_CXX_COMPILER=i686-w64-mingw32-clang++ \
+          -C "${MONOREPO_ROOT}/libcxx/cmake/caches/MinGW.cmake"
+    echo "+++ Running the libc++ tests"
+    ${NINJA} -vC "${BUILD_DIR}" check-cxx
+;;
 aix)
     export CC=ibm-clang
     export CXX=ibm-clang++_r
     clean
     generate-cmake -C "${MONOREPO_ROOT}/libcxx/cmake/caches/AIX.cmake" \
                    -DLIBCXX_TEST_CONFIG="ibm-libc++-shared.cfg.in" \
                    -DLIBCXXABI_TEST_CONFIG="ibm-libc++abi-shared.cfg.in" \
                    -DLLVM_ENABLE_RUNTIMES="libcxx;libcxxabi"