Add PRIVATE keyword in target_link_libraries to prevent CMake Error on Windows.
While trying to compile llvm/clang on Windows, the following CMake error occurred. The reason is a missing PUBLIC/PRIVATE/INTERFACE keyword in target_link_libraries.
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CMake Error at utils/KillTheDoctor/CMakeLists.txt:5 (target_link_libraries):
The keyword signature for target_link_libraries has already been used with the target "KillTheDoctor". All uses of target_link_libraries with a target must be either all-keyword or all-plain. The uses of the keyword signature are here: * cmake/modules/AddLLVM.cmake:771 (target_link_libraries)
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