Move AST utilities to a subdirectory

Move AST related utilities to compiler/translator/tree_util.

BUG=angleproject:2409
TEST=angle_unittests

Change-Id: I7567c2f6f2710292029263257c7ac26e2a144ac8
Reviewed-on: https://chromium-review.googlesource.com/966032
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
diff --git a/src/compiler/translator/OutputTree.cpp b/src/compiler/translator/OutputTree.cpp
index 00b40b1..8d2c127 100644
--- a/src/compiler/translator/OutputTree.cpp
+++ b/src/compiler/translator/OutputTree.cpp
@@ -4,8 +4,8 @@
 // found in the LICENSE file.
 //
 
-#include "compiler/translator/IntermTraverse.h"
 #include "compiler/translator/SymbolTable.h"
+#include "compiler/translator/tree_util/IntermTraverse.h"
 
 namespace sh
 {