Fix some "not all control paths return" warnings; NFC

clang-tools-extra/clang-doc/Representation.cpp

@@ -147,6 +147,7 @@ mergeInfos(std::vector<std::unique_ptr<Info>> &Values) {
     return llvm::createStringError(llvm::inconvertibleErrorCode(),
                                    "unexpected info type");
   }
+  llvm_unreachable("unhandled enumerator");
 }
 
 bool CommentInfo::operator==(const CommentInfo &Other) const {

clang-tools-extra/clang-doc/Serialize.cpp

@@ -392,6 +392,7 @@ std::string serialize(std::unique_ptr<Info> &I) {
   case InfoType::IT_default:
     return "";
   }
+  llvm_unreachable("unhandled enumerator");
 }
 
 static void parseFullComment(const FullComment *C, CommentInfo &CI) {