clang-tidy: remove unnecessary calls to std::string::c_str(), redundant void...

clang-tidy: remove unnecessary calls to std::string::c_str(), redundant void arguments and explicit calls to String upon return

1. https://clang.llvm.org/extra/clang-tidy/checks/readability-redundant-string-cstr.html
2. https://clang.llvm.org/extra/clang-tidy/checks/modernize-redundant-void-arg.html
3. https://clang.llvm.org/extra/clang-tidy/checks/modernize-return-braced-init-list.html