Summary:
Fixes https://github.com/facebook/folly/issues/1449

The current public VS releases (16.3 up to 16.7) have builtins support. In commit https://github.com/facebook/folly/commit/7f1bda25627503ca8af2536cc81298a7d4688375 the #ifdef that was gating the definition of builtins was removed, as 16.8 will remove it. However by removign this gate, it makes current VS versions unable to build folly which breaks projects like react-native-windows.

Pull Request resolved: https://github.com/facebook/folly/pull/1450

Reviewed By: yfeldblum

Differential Revision: D23702071

Pulled By: Orvid

fbshipit-source-id: 6e05f63293e68783b6caae68cc5bd80ad3679766