Summary:
- Prior to commit 8cb615a2
  (Differential Revision: D3394612), `Range:size()` had an assert checking
  that the beginning iterator of the range is less than or equal to the end
  iterator. Unfortunately, due to GCC bug 71448, the `assert` was not allowed
  if we wanted `size()` to remain a `constexpr` function.
- This bug is no longer an issue with our current supported versions of GCC.
  As such, add the assert back in.
Pull Request resolved: https://github.com/facebook/folly/pull/1155

Reviewed By: ericniebler

Differential Revision: D15641627

Pulled By: yfeldblum

fbshipit-source-id: 0677053c18fa55aaa4727947c241f0675362861b