[CMake] Restructure how Clang, Polly and other external projects get included.

While making lld build under the tools directory I decided to refactor how this
works.

There is now a macro, add_llvm_external_project, which takes the name of the
expected subdirectory. This sets up two CMake options.

 * LLVM_EXTERNAL_${NAME}_SOURCE_DIR
     This is the path to the source. It defaults to
     ${CMAKE_CURRENT_SOURCE_DIR}/${name}.
 * LLVM_EXTERNAL_${NAME}_BUILD
     Enable and disable building the tool as part of LLVM.

I chose LLVM_EXTERNAL_${NAME} as a prefix so they all show up together in the
GUI.

llvm-svn: 155654