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

test/Object/X86/lit.local.cfg

@@ -1,13 +1,6 @@
 config.suffixes = ['.test']
 
-def getRoot(config):
-    if not config.parent:
-        return config
-    return getRoot(config.parent)
-
-root = getRoot(config)
-
-targets = set(root.targets_to_build.split())
+targets = set(config.root.targets_to_build.split())
 if not 'X86' in targets:
     config.unsupported = True