Make a major restructuring of the clang tree: introduce a top-level

lib dir and move all the libraries into it.  This follows the main
llvm tree, and allows the libraries to be built in parallel.  The
top level now enforces that all the libs are built before Driver,
but we don't care what order the libs are built in.  This speeds
up parallel builds, particularly incremental ones.

llvm-svn: 48402