deduction guides for std::reference_wrapper

Header: <functional>

One deduction guide is provided for std::reference_wrapper to support deduction of the sole class template parameter.

# Declarations

template< typename T >
reference_wrapper( T& ) -> reference_wrapper<T>;

(since C++17)

# Example

This section is incompleteReason: no example

# Defect reports

DRApplied toBehavior as publishedCorrect behavior
LWG 2981C++17a redundant deduction guide from reference_wrapper was providedremoved
LWG 2993C++17defect resolution removed a constructor used for class template argument deductionadded deduction guide to compensate