[clang-tidy] Add return value for non-assert builds.

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@251262 91177308-0d34-0410-b5e6-96231b3b80d8

clang-tidy/readability/ImplicitBoolCastCheck.cpp

@@ -84,6 +84,7 @@ getZeroLiteralToCompareWithForGivenType(CastKind CastExpressionKind,
   default:
     assert(false && "Unexpected cast kind");
   }
+  return "";
 }
 
 bool isUnaryLogicalNotOperator(const Stmt *Statement) {