Remove use of "dummy"

Replace "dummy" with "placeholder".

Change-Id: I9af7f56c93b49c1a6428414601af1dec475ca3b8
diff --git a/src/code-buffer-vixl.h b/src/code-buffer-vixl.h
index a43c584..9a1efd4 100644
--- a/src/code-buffer-vixl.h
+++ b/src/code-buffer-vixl.h
@@ -164,8 +164,8 @@
     *has_grown = is_full;
   }
   void EnsureSpaceFor(size_t amount) {
-    bool dummy;
-    EnsureSpaceFor(amount, &dummy);
+    bool placeholder;
+    EnsureSpaceFor(amount, &placeholder);
   }
 
  private: