Simplify _fs UDL in FixedString.h (#1149)
Simplify _fs UDL in FixedString.h (#1149)

Summary:
- Simplify `_fs` user defined literal by removing the preprocessor branch.
- Since Folly only supports C++14 now, `#if __cplusplus >= 201402L` will always
  evaluate to true.
Pull Request resolved: https://github.com/facebook/folly/pull/1149

Reviewed By: ericniebler

Differential Revision: D15638288

Pulled By: yfeldblum

fbshipit-source-id: b690e51cbe8ba57baeb8704722e9f126f15e9d4a

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly