[clang-tidy] fix performance issues

Found with performance*

Signed-off-by: Rosen Penev <rosenp@gmail.com>

Author: neheb

src/jsontestrunner/main.cpp

@@ -111,8 +111,8 @@ static void printValueTree(FILE* fout, Json::Value& value,
     Json::Value::Members members(value.getMemberNames());
     std::sort(members.begin(), members.end());
     Json::String suffix = *(path.end() - 1) == '.' ? "" : ".";
-    for (auto name : members) {
-      printValueTree(fout, value[name], path + suffix + name);
+    for (const auto& name : members) {
+      printValueTree(fout, value[name], path + suffix.append(name));
     }
   } break;
   default:

src/test_lib_json/main.cpp

@@ -3286,11 +3286,11 @@ struct CharReaderAllowDropNullTest : JsonTest::TestCase {
     return [=](const Value& root) { JSONTEST_ASSERT_EQUAL(root, v); };
   }
 
-  static ValueCheck objGetAnd(std::string idx, ValueCheck f) {
+  static ValueCheck objGetAnd(const std::string& idx, const ValueCheck& f) {
     return [=](const Value& root) { f(root.get(idx, true)); };
   }
 
-  static ValueCheck arrGetAnd(int idx, ValueCheck f) {
+  static ValueCheck arrGetAnd(int idx, const ValueCheck& f) {
     return [=](const Value& root) { f(root[idx]); };
   }
 };