Update clang to r339409.
Change-Id: I800772d2d838223be1f6b40d490c4591b937fca2
diff --git a/linux-x64/clang/include/llvm/ADT/GraphTraits.h b/linux-x64/clang/include/llvm/ADT/GraphTraits.h
index 27c647f..d39b50f 100644
--- a/linux-x64/clang/include/llvm/ADT/GraphTraits.h
+++ b/linux-x64/clang/include/llvm/ADT/GraphTraits.h
@@ -25,6 +25,13 @@
// GraphTraits - This class should be specialized by different graph types...
// which is why the default version is empty.
//
+// This template evolved from supporting `BasicBlock` to also later supporting
+// more complex types (e.g. CFG and DomTree).
+//
+// GraphTraits can be used to create a view over a graph interpreting it
+// differently without requiring a copy of the original graph. This could
+// be achieved by carrying more data in NodeRef. See LoopBodyTraits for one
+// example.
template<class GraphType>
struct GraphTraits {
// Elements to provide: