CI: only clone/fetch the parts of LLVM monorepo that we need

This ends up being *much* faster, noticeably speeding up these jobs.

.github/libcxx-setup.sh

@@ -3,7 +3,12 @@
 set -e
 
 # Checkout LLVM sources
-git clone --depth=1 --branch llvmorg-19.1.6 https://github.com/llvm/llvm-project.git llvm-project
+git clone --filter=blob:none --depth=1 --branch llvmorg-19.1.6 --no-checkout https://github.com/llvm/llvm-project.git llvm-project
+cd llvm-project
+git sparse-checkout set --cone
+git checkout llvmorg-19.1.6
+git sparse-checkout set cmake llvm/cmake runtimes libcxx libcxxabi
+cd ..
 
 ## Setup libc++ options
 if [ -z "$BUILD_32_BITS" ]; then
@@ -20,6 +25,9 @@ cmake -DCMAKE_C_COMPILER=${CC}                  \
       -DLLVM_USE_SANITIZER=${LIBCXX_SANITIZER}  \
       -DLLVM_BUILD_32_BITS=${BUILD_32_BITS}     \
       -DLIBCXXABI_USE_LLVM_UNWINDER=OFF         \
+      -DLLVM_INCLUDE_TESTS=OFF                  \
+      -DLIBCXX_INCLUDE_TESTS=OFF                \
+      -DLIBCXX_INCLUDE_BENCHMARKS=OFF           \
       -DLLVM_ENABLE_RUNTIMES='libcxx;libcxxabi' \
       -G "Unix Makefiles" \
       ../llvm-project/runtimes/