[cmake] llvm runtimes on Windows are not using clang-cl

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Hello,

I’ve been trying to build libcxx on Windows using the -DLLVM_INCLUDE_RUNTIMES=On -DLLVM_ENABLE_RUNTIMES=”libcxx;libcxxabi” options.

However, it seems that LLVMExternalProjectUtils.cmake insists on using clang instead of clang-cl on Windows, causing CMake to error out.

This patch added support for using clang-cl: https://github.com/llvm/llvm-project/commit/dc1c41fb6f096d5c3df5d58e2a6f49bb915e1c31

However, it seems that none of the CMake files under llvm-project/llvm/runtimes are passing DCMAKE_SYSTEM_NAME=Windows when they call the functions mentioned in the patch above, a requirement for that patch to have any effects. No follow-up patches seem to have been submitted to exploit the newly added feature.

Has anyone experienced similar problems?

Thanks!