Add back the top-level target for 'llvm-config' and the dependency edge

to force it to build after all library targets so it has complete
dependency information. This should fix broken 'make install' with
CMake.

This is a partial revert of r143540, but it doesn't revert the most
important part of that change: removing the dependency edge from LLVM
tools to the llvm-config script.

llvm-svn: 143548