Cut the ScopeGuard alias now that we have auto

Cut the ScopeGuard alias now that we have auto

Summary:
[Folly] Cut the `ScopeGuard` alias now that we have `auto`.

This form works because of hidden lifetime extension:
```lang=c++
folly::ScopeGuard guard = folly::makeGuard([] { /*...*/ });
//  ...
//  guard falls out of scope
```
But this form would not work correctly:
```lang=c++
folly::ScopeGuard guard = folly::makeGuard([] { /*...*/ });
std::async(std::launch::async, [guard = std::move(guard)] {});
```
Because `folly::ScopeGuard` is an rvalue-reference-to-base.
We have `auto`, so just remove `folly::ScopeGuard`. This form works correctly:
```lang=c++
auto guard = folly::makeGuard([] { /*...*/ });
std::async(std::launch::async, [guard = std::move(guard)] {});
```

Reviewed By: igorsugak

Differential Revision: D6690070

fbshipit-source-id: 54e32b300d36fce4eb95a59f1828819afe312ec0