CMake builds clang.

If you put the clang source dir under the tools/ LLVM *source* dir, it
will be added to the build, just as if it were another llvm tool.

Maybe you'll need to re-invoke cmake on your current build dir so it
notices the presence of clang on the source dir.

Having cmake support for clang adds some maintenance work in the sense
that now I have to watch LLVM and clang for file structure
modifications. As there are long periods of time when I don't touch LLVM
at all, it would be great if you reflect your changes on the file list
inside the corresponding CMakeLists.txt when you add, remove or rename a
.cpp file. If the modification is more complex, or you notice that the
cmake build is broken, sending me a reminder would be much appreciated.