Fix no-assert compiles.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@182569 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Format/UnwrappedLineParser.h b/lib/Format/UnwrappedLineParser.h
index 842b108..70e860c 100644
--- a/lib/Format/UnwrappedLineParser.h
+++ b/lib/Format/UnwrappedLineParser.h
@@ -127,7 +127,10 @@
   // the UnwrappedLineParser once we switch to generate all tokens
   // up-front.
   virtual unsigned getPosition() { return 0; }
-  virtual FormatToken setPosition(unsigned Position) { assert(false); }
+  virtual FormatToken setPosition(unsigned Position) {
+    assert(false);
+    return FormatToken();
+  }
 };
 
 class UnwrappedLineParser {