Rework site config for cmake to be generated at configure time, and only pass
the 'build_config' value in at runtime using the new lit runtime user parameter
feature.

This simplifies things and drops a dependency on 'sed', FWIW.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@86421 91177308-0d34-0410-b5e6-96231b3b80d8