[clang] [libcxx] [Cygwin][MinGW] Internal class in explicitly-instantiation-declarated template should be instantiated (PR #140145)

[via cfe-commits]

jeremyd2019 wrote:

They've failed linking things due to missing `std::istream::sentry` - I've started a new run https://github.com/jeremyd2019/llvm-mingw/actions/runs/15078849158 with 
```
#    if defined(__MINGW32__) || defined(__CYGWIN__)
extern template class _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS basic_istream<char>::sentry;
#    endif
```

rather than explicitly listing the member(s).

https://github.com/llvm/llvm-project/pull/140145