Clang build system

Right now, Clang can only be built inside of a full LLVM checkout. This
seems kind of unnecessary, since it's a separate tool. What would it
take to add an independent build system to Clang, so that it could be
built by itself (assuming a working LLVM install elsewhere on the
system, I guess)?


WFIW "make clang-only" in the llvm directory builds only clang.

We have <> tracking improvements in this area.