[lit] Allow config files to pass arbitrary values to child configs.

- This aligns with how existing test suites end up wanting to use the local
   config files, conceptually it makes sense to consider them to be inherited.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189885 91177308-0d34-0410-b5e6-96231b3b80d8

utils/lit/tests/Inputs/discovery/lit.cfg

@@ -9,3 +9,6 @@ config.test_format = lit.formats.ShTest()
 #
 #config.test_source_root = None
 #config.test_exec_root = None
+
+# Check that arbitrary config values are copied (tested by subdir/lit.local.cfg).
+config.an_extra_variable = False

utils/lit/tests/Inputs/discovery/subdir/lit.local.cfg

@@ -1 +1,4 @@
 config.suffixes = ['.py']
+
+# Check that the arbitrary config values in our parent was inherited.
+assert hasattr(config, 'an_extra_variable')