[libc++] Disallow running the libc++ benchmarks in standalone builds

We are trying to remove duplication of third-party code in
https://reviews.llvm.org/D112012, which will move the Google
Benchmark code outside of the `libcxx/` directory. That breaks
running the benchmarks in the Standalone build. Since we have
deprecated the Standalone build anyway, this patch just removes
support for the benchmark in Standalone mode until we remove that
mode entirely.

Differential Revision: https://reviews.llvm.org/D113503

GitOrigin-RevId: 4e70b50b74dba909f1d8277e26896d9a0d23d428
diff --git a/benchmarks/CMakeLists.txt b/benchmarks/CMakeLists.txt
index 67e643c..8758c6b 100644
--- a/benchmarks/CMakeLists.txt
+++ b/benchmarks/CMakeLists.txt
@@ -3,6 +3,12 @@
   return()
 endif()
 
+if (LIBCXX_STANDALONE_BUILD)
+  message(WARNING "The libc++ benchmarks are not available in a standalone build. Please migrate to an official build "
+                  "as documented in https://libcxx.llvm.org/BuildingLibcxx.html.")
+  return()
+endif()
+
 include(ExternalProject)
 include(CheckCXXCompilerFlag)
 
diff --git a/utils/ci/run-buildbot b/utils/ci/run-buildbot
index b36f6d7..579721f 100755
--- a/utils/ci/run-buildbot
+++ b/utils/ci/run-buildbot
@@ -520,6 +520,7 @@
           -DCMAKE_INSTALL_PREFIX="${INSTALL_DIR}" \
           -DLLVM_PATH="${MONOREPO_ROOT}/llvm" \
           -DLIBCXX_CXX_ABI=libcxxabi \
+          -DLIBCXX_INCLUDE_BENCHMARKS=OFF \
           -DLIBCXX_CXX_ABI_INCLUDE_PATHS="${MONOREPO_ROOT}/libcxxabi/include" \
           -DLIBCXX_CXX_ABI_LIBRARY_PATH="${BUILD_DIR}/libcxxabi/lib"