Address MSVC C4686 in folly/lang/Pretty.h

Summary:
[Folly] Address MSVC C4686 in `folly/lang/Pretty.h`.

Resolves:

```
folly\lang\pretty.h(119): warning C4686: 'folly::detail::pretty_name_zarray<T>::zarray_': possible change in behavior, change in UDT return calling convention
```

Reviewed By: Orvid

Differential Revision: D14214290

fbshipit-source-id: c5e0ebd7c100413737bdcba157ec8b5ce42895de

folly/lang/Pretty.h

@@ -108,7 +108,7 @@ struct pretty_name_zarray {
   static constexpr auto raw = pretty_raw<T>();
   static constexpr auto info = pretty_parse(raw.data);
   static constexpr auto size = info.e - info.b;
-  static constexpr pretty_carray<size + 1> zarray_() {
+  static constexpr auto zarray_() {
     pretty_carray<size + 1> data{};
     for (std::size_t i = 0; i < size; ++i) {
       data.data[i] = raw.data[info.b + i];