[CMake] Projects supported via LLVM_EXTERNAL_*_SOURCE_DIR need to be explicitly specified.

One part of my refactoring from r242705 is untenable due to how CMake caches variables. There is no way other than caching to allow variables to be set in one directory and globally readable, but we really don't want to cache the temporary value marking that a directory has already been included.

llvm-svn: 242793