[lit] Allow config files to pass arbitrary values to child configs.

 - This aligns with how existing test suites end up wanting to use the local
   config files, conceptually it makes sense to consider them to be inherited.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189885 91177308-0d34-0410-b5e6-96231b3b80d8