CMake: build llvm-config on Windows.

It was previously not being built on Windows because the cmake file relied
on a sed script to generate a .in file that llvm-config needs.

By using cmake's configure_file function, we can get rid off the sed hack,
and also have this work on Windows.

Differential Revision: http://llvm-reviews.chandlerc.com/D1481

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189125 91177308-0d34-0410-b5e6-96231b3b80d8