ci: use LLVM libcxx 21.1.5

Author: fanquake

ci/test/01_base_install.sh

@@ -57,7 +57,7 @@ if [ -n "$PIP_PACKAGES" ]; then
 fi
 
 if [[ -n "${USE_INSTRUMENTED_LIBCPP}" ]]; then
-  ${CI_RETRY_EXE} git clone --depth=1 https://github.com/llvm/llvm-project -b "llvmorg-21.1.1" /llvm-project
+  ${CI_RETRY_EXE} git clone --depth=1 https://github.com/llvm/llvm-project -b "llvmorg-21.1.5" /llvm-project
 
   cmake -G Ninja -B /cxx_build/ \
     -DLLVM_ENABLE_RUNTIMES="libcxx;libcxxabi;libunwind" \