Address post-review comment from dblakie.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@182732 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Format/UnwrappedLineParser.h b/lib/Format/UnwrappedLineParser.h
index bf3db1e..c738ef1 100644
--- a/lib/Format/UnwrappedLineParser.h
+++ b/lib/Format/UnwrappedLineParser.h
@@ -120,7 +120,7 @@
   // up-front.
   virtual unsigned getPosition() { return 0; }
   virtual FormatToken setPosition(unsigned Position) {
-    assert(false);
+    llvm_unreachable("Interface in transition; do not call!");
     return FormatToken();
   }
 };