[libc++] Add Unstable ABI CI run

Reviewed By: ldionne, #libc, Mordante

Spies: mgorny, Mordante, libcxx-commits, arichardson

Differential Revision: https://reviews.llvm.org/D118725

libcxx/cmake/caches/Generic-abi-unstable.cmake

@@ -0,0 +1 @@
+set(LIBCXX_ABI_UNSTABLE ON CACHE BOOL "")

libcxx/utils/ci/buildkite-pipeline.yml

@@ -465,6 +465,19 @@ steps:
           limit: 2
     timeout_in_minutes: 120
 
+  - label: "Unstable ABI"
+    command: "libcxx/utils/ci/run-buildbot generic-abi-unstable"
+    artifact_paths:
+      - "**/test-results.xml"
+    agents:
+      queue: "libcxx-builders"
+      os: "linux"
+    retry:
+      automatic:
+        - exit_status: -1  # Agent was lost
+          limit: 2
+    timeout_in_minutes: 120
+
   - label: "No experimental features"
     command: "libcxx/utils/ci/run-buildbot generic-no-experimental"
     artifact_paths:

libcxx/utils/ci/run-buildbot

@@ -397,6 +397,13 @@ generic-no-experimental)
                    -DLIBUNWIND_TEST_CONFIG="llvm-libunwind-shared.cfg.in"
     check-runtimes
 ;;
+generic-abi-unstable)
+    clean
+    generate-cmake -C "${MONOREPO_ROOT}/libcxx/cmake/caches/Generic-abi-unstable.cmake" \
+                   -DLIBCXX_TEST_CONFIG="llvm-libc++-shared.cfg.in" \
+                   -DLIBUNWIND_TEST_CONFIG="llvm-libunwind-shared.cfg.in"
+    check-runtimes
+;;
 apple-system)
     clean