Currently, setting -DLLVM_ENABLE_LLD=ON on windows also requires setting -DCMAKE_LINKER=lld-link.exe. This is both misleading and redundant. Fix this by trying to find llvm-link.exe when -DLLVM_ENABLE_LLD=ON is set and CMAKE_LINKER is not, and aborting otherwise. Differential Revision: https://reviews.llvm.org/D140534
See docs/CMake.html for instructions on how to build LLVM with CMake.