[lit] Inject the lit specific config object as 'lit_config' when loading config files.

[lit] Inject the lit specific config object as 'lit_config' when loading config files.

 - Injecting it as 'lit' is gross, since that name should be used to refer to
   the actual package. For now both are available so it is possibly to cleanup
   test config files incrementally.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188039 91177308-0d34-0410-b5e6-96231b3b80d8