Remove forward declarations (#48461)

Summary:
Pull Request resolved: #48461

Remove forward class or struct declarations.

Changelog: [Internal]

```
$ sl log --stat tools/api/ReactNativeCPP.api
 xplat/js/react-native-github/tools/api/ReactNativeCPP.api |  637
 1 files changed, 96 insertions(+), 541 deletions(-)
```

Reviewed By: cipolleschi

Differential Revision: D67763260

fbshipit-source-id: 396314be9cb6153f6cff1348aa596d4a5b61fbe7

Author: blakef