Change ClangTidy unit tests to run in C++20 mode instead of C++11.

clang-tools-extra/unittests/clang-tidy/ClangTidyTest.h

runCheckOnCode(StringRef Code, std::vector<ClangTidyError> *Errors = nullptr,
   Args.push_back("-fno-delayed-template-parsing");
   std::string extension(
       std::string(llvm::sys::path::extension(Filename.str())));
   if (extension == ".m" || extension == ".mm") {
     Args.push_back("-fobjc-abi-version=2");
     Args.push_back("-fobjc-arc");
   }
   if (extension == ".cc" || extension == ".cpp" || extension == ".mm") {
-    Args.push_back("-std=c++11");
+    Args.push_back("-std=c++20");
   }
   Args.push_back("-Iinclude");
   Args.insert(Args.end(), ExtraArgs.begin(), ExtraArgs.end());
   Args.push_back(Filename.str());
 
   ast_matchers::MatchFinder Finder;
   llvm::IntrusiveRefCntPtr<llvm::vfs::InMemoryFileSystem> InMemoryFileSystem(
       new llvm::vfs::InMemoryFileSystem);