[CMake] Automatically pick up subdirectories in llvm/tools as 'external...

[CMake] Automatically pick up subdirectories in llvm/tools as 'external projects' if they contain a 'CMakeLists.txt' file.

Allow CMake to pick up external projects in llvm/tools without the need to modify the "llvm/tools/CMakeLists.txt" file.
This makes it easier to work with projects that live in other repositories, without needing to specify each one in "llvm/tools/CMakeLists.txt".

llvm-svn: 188921