[CMake] Unbreak add_llvm_external_project when external projects are specified.

LLVM_EXTERNAL_*_SOURCE_DIR is reset as PATH with set(CACHE PATH).
Then the CACHE PATH variable, LLVM_EXTERNAL_*_SOURCE_DIR, is normalized as
${CMAKE_SOURCE_DIR}/${path_var} if ${path_var} is relative.

llvm-svn: 242120