[libc++][format] Don't treat a closing '}' as part of format-spec (#81305)

This allows:
```
std::println("{}>42", std::thread::id{});
std::println("{}>42", std::span<int>{});
std::println("{}>42", std::pair{42, "Hello"sv});
std::println("{:}>42", std::thread::id{});
std::println("{:}>42", std::span<int>{});
std::println("{:}>42", std::pair{42, "Hello"sv});
```
to compile and run.