The tool used the 'old' LLVM build information (LLVMBuild.txt),
which caused linking errors in:
https://lab.llvm.org/buildbot/#/builders/177/builds/10125
https://lab.llvm.org/buildbot/#/builders/196/builds/19699
Those configurations use:
LLVM_BUILD_LLVM_DYLIB=ON LLVM_LINK_LLVM_DYLIB=ON
Update the CMake configuration to support the new LLVM build
system that uses only CMakeLists.txt.