Continue cleanup of LIT, getting rid of the remaining artifacts from dejagnu

* Removed test/lib/llvm.exp - it is no longer needed 
* Deleted the dg.exp reading code from test/lit.cfg. There are no dg.exp files
  left in the test suite so this code is no longer required. test/lit.cfg is
  now much shorter and clearer 
* Removed a lot of duplicate code in lit.local.cfg files that need access to
  the root configuration, by adding a "root" attribute to the TestingConfig
  object. This attribute is dynamically computed to provide the same
  information as was previously provided by the custom getRoot functions. 
* Documented the config.root attribute in docs/CommandGuide/lit.pod

llvm-svn: 153408