[CMake] Put back the add_llvm_external_project() calls for specific projects.

This allows setting-up the LLVM_EXTERNAL_* CMake variables that some people are using,
e.g. to set the source directory of the project in a different place.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189415 91177308-0d34-0410-b5e6-96231b3b80d8