CMake Errors: /LLVMBuild.txt and others do not exist

Question

I'm trying to build clang on Windows7 following the instructions here:

cmake -DLLVM_ENABLE_PROJECTS=clang -G "Visual
Studio 15 2017" -A x64 -Thost=x64 ..\llvm

And there are errors like this(traceback):

CMake Error: File /cygdrive/c/llvm-project
/llvm/LLVMBuild.txt does not exist.
CMake Error at C:/llvm-project/build
/LLVMBuild.cmake:27 (configure_file):
configure_file Problem configuring file
Call Stack (most recent call first):
CMakeLists.txt:720 (include)

Even when the files are there.