Explicitly delete fbstring_core copy-assignment (#1123)
Explicitly delete fbstring_core copy-assignment (#1123)

Summary:
- Replace `fbstring_core` private copy assignment special member function
  with an explicit `=delete` equivalent.
Pull Request resolved: https://github.com/facebook/folly/pull/1123

Reviewed By: Orvid

Differential Revision: D15216597

Pulled By: yfeldblum

fbshipit-source-id: 319109f739d972e9efc73a7dbc2dd6ae401b18fc

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