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

utils/lit/lit/TestingConfig.py

@@ -114,3 +114,9 @@ class TestingConfig:
             # files. Should we distinguish them?
             self.test_source_root = str(self.test_source_root)
         self.excludes = set(self.excludes)
+
+    @property
+    def root(self):
+        """root attribute - The root configuration for the test suite."""
+        return self if self.parent is None else self.parent.root
+