[libc++][ci] Enable modules in the Runtimes build

[ldionne]

Summary

The runtimes build has assertions enabled, which is necessary to catch
some of the modules-related issues we've been seeing recently. This
patch enables testing with modules in the runtimes build so as to cover
those cases.

In the future, a better solution would be to systematically use versions
of Clang that have assertions enabled. However, the Clangs we release
currently don't have assertions enabled by default, which causes a
challenge for the CI (we could try to build our own Clang from ToT with
assertions in the CI, but that poses some problems).

libcxx/utils/ci/run-buildbot

 ;;
 runtimes-build)
     export CC=clang
     export CXX=clang++
 
     clean
 
     echo "--- Generating CMake"
+    # TODO: We currently enable modules and assertions in the runtimes build
+    #       because that provides coverage for some specific Clang failures
+    #       we've been seeing recently, however it would be better to instead
+    #       run all CI configurations against a Clang that has assertions enabled.
     ${CMAKE} \
           -S "${MONOREPO_ROOT}/llvm" \
           -B "${BUILD_DIR}" \
           -GNinja -DCMAKE_MAKE_PROGRAM="${NINJA}" \
           -DCMAKE_BUILD_TYPE=RelWithDebInfo \
           -DCMAKE_INSTALL_PREFIX="${INSTALL_DIR}" \
           -DLLVM_ENABLE_PROJECTS="clang" \
           -DLLVM_ENABLE_RUNTIMES="libcxx;libcxxabi" \
           -DLLVM_RUNTIME_TARGETS="x86_64-unknown-linux-gnu" \
-          -DLLVM_ENABLE_ASSERTIONS=ON
+          -DLLVM_ENABLE_ASSERTIONS=ON \
+          -C "${MONOREPO_ROOT}/libcxx/cmake/caches/Generic-modules.cmake"
 
     echo "+++ Running the libc++ and libc++abi tests"
     ${NINJA} -C "${BUILD_DIR}" check-runtimes
 
     echo "--- Installing libc++ and libc++abi to a fake location"
     ${NINJA} -C "${BUILD_DIR}" install-cxx install-cxxabi
 ;;
 legacy-test-config)