Compile libc++ as LLVM IR

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https://github.com/llvm/llvm-project/tree/main/libcxx

[ds5678]

Run this in llvm-project.

cmake -S runtimes -B build -G "Ninja" -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ -DCMAKE_C_FLAGS="-DNOMINMAX" -DCMAKE_CXX_FLAGS="-DNOMINMAX" -DCMAKE_EXPORT_COMPILE_COMMANDS=ON -DLLVM_ENABLE_RUNTIMES="libcxx" -DLIBCXXABI_USE_LLVM_UNWINDER=OFF -DLIBCXX_CXX_ABI=vcruntime

Then use LargeProjectCompiler on it.