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

[ldionne]

Summary

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.

[Mordante]

Good the know this will soon stop working.
LGTM!

[ldionne]

Shipping despite the CI failure since this is a known issue and I've fixed it since then.

libcxx/benchmarks/CMakeLists.txt

 if (CMAKE_VERSION VERSION_LESS 3.17)
   message(WARNING "The libc++ benchmarks won't be available because the version of CMake is too old to support them.")
   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)
 
 #==============================================================================
 # Build Google Benchmark for libc++
 #==============================================================================
 
 set(CMAKE_FOLDER "${CMAKE_FOLDER}/Benchmarks")

libcxx/utils/ci/run-buildbot

legacy-standalone)
     ${CMAKE} \
           -S "${MONOREPO_ROOT}/libcxx" \
           -B "${BUILD_DIR}/libcxx" \
           -GNinja -DCMAKE_MAKE_PROGRAM="${NINJA}" \
           -DCMAKE_BUILD_TYPE=RelWithDebInfo \
           -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"
 
     ${CMAKE} \
           -S "${MONOREPO_ROOT}/libcxxabi" \
           -B "${BUILD_DIR}/libcxxabi" \
           -GNinja -DCMAKE_MAKE_PROGRAM="${NINJA}" \
           -DCMAKE_BUILD_TYPE=RelWithDebInfo \