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

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@143548 91177308-0d34-0410-b5e6-96231b3b80d8