Fix windows build failure when using Empty struct with for_each_ordinal
Fix windows build failure when using Empty struct with for_each_ordinal

Summary:
Following code won't built in MSVC

```
int unused[] = {};
```

Since array size can't be 0: https://godbolt.org/z/xPnGG3GPa

It works in clang/gcc as an extension: https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html, but this behavior is not standard conforming.

Reviewed By: vitaut

Differential Revision: D41016136

fbshipit-source-id: 9791d7a285ad4a5ea3984e96dda5cc556593ba86

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