Drop dead return after llvm_unreachable. NFC.

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@251279 91177308-0d34-0410-b5e6-96231b3b80d8

clang-tidy/readability/ImplicitBoolCastCheck.cpp

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