libstdc++: Simplify std::forward_list assignment using 'if constexpr'

Use diagnostic pragmas to allow using `if constexpr` in C++11 mode, so
that we don't need to use tag dispatching.

The unused member functions are preserved for the purposes of explicit
instantiations. The _M_assign function template can be removed, because
member function templates aren't instantiated by explicit instantiations
anyway.

libstdc++-v3/ChangeLog:

	* include/bits/forward_list.h (operator=(forward_list&&)): Use
	if constexpr instead of dispatching to _M_move_assign.
	(assign(InputIterator, InputIterator)): Use if constexpr instead
	of dispatching to _M_assign.
	(assign(size_type, const T&)): Use if constexpr instead of
	dispatching to _M_assign_n.
	(_M_move_assign, _M_assign_n): Do not define for versioned
	namespace.
	(_M_assign): Remove.

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