folly: fix clang's -Wundefined-var-template

folly: fix clang's -Wundefined-var-template

Summary:
[temp] (14)/6:
> A function template, member function of a class template, variable template, or static data member of a class template shall be defined in every translation unit in which it is implicitly instantiated (14.7.1) unless the corresponding specialization is explicitly instantiated (14.7.2) in some translation unit; no diagnostic is required.

`-Wundefined-var-template` warns on any implicit instantiations that are needed but could not be performed because the definition is not available. In particular, for valid code, this warns on templates/temploids which have their definition and all relevant explicit instantiations tucked away in some source file (but for which no explicit instantiation declarations are provided in the relevant header file) - used a few times in folly. This seems a bad style, the static data member template should either be defined in the header file or should be explicitly instantiated in each .cpp file.

Reviewed By: igorsugak

Differential Revision: D3270439

fbshipit-source-id: bf305fde3af575a265d65a0ae0bf95db5597587f