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





git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@153408 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/docs/CommandGuide/lit.pod b/docs/CommandGuide/lit.pod
index e0b1a8c..5eeee11 100644
--- a/docs/CommandGuide/lit.pod
+++ b/docs/CommandGuide/lit.pod
@@ -301,6 +301,9 @@
 B<parent> The parent configuration, this is the config object for the directory
 containing the test suite, or None.
 
+B<root> The root configuration. This is the top-most B<lit> configuration in
+the project.
+
 B<on_clone> The config is actually cloned for every subdirectory inside a test
 suite, to allow local configuration on a per-directory basis. The I<on_clone>
 variable can be set to a Python function which will be called whenever a