[lit] Fix running tests that require 'examples'.

Differential Revision: https://reviews.llvm.org/D46514
Patch by Nikolai Kosjar.

llvm-svn: 331786

clang/test/lit.cfg.py

@@ -63,6 +63,7 @@ tools = [
 ]
 
 if config.clang_examples:
+    config.available_features.add('examples')
     tools.append('clang-interpreter')
 
 llvm_config.add_tool_substitutions(tools, tool_dirs)