[analyzer] CastValueChecker: Try to fix the buildbots

llvm-svn: 369609
diff --git a/clang/lib/StaticAnalyzer/Checkers/CastValueChecker.cpp b/clang/lib/StaticAnalyzer/Checkers/CastValueChecker.cpp
index e7472ab..1580f7f 100644
--- a/clang/lib/StaticAnalyzer/Checkers/CastValueChecker.cpp
+++ b/clang/lib/StaticAnalyzer/Checkers/CastValueChecker.cpp
@@ -110,7 +110,7 @@
   Object = Object->IgnoreParenImpCasts();
 
   return C.getNoteTag(
-      [=] {
+      [=]() -> std::string {
         SmallString<128> Msg;
         llvm::raw_svector_ostream Out(Msg);