[PATCH] Simplify reference_wrapper nested typedefs

[Jonathan Wakely]

More <functional> refactoring. This uses the void_t "detection idiom"
to test for the members, with fewer templates and fewer lines of code.

There are now no more uses of _GLIBCXX_HAS_NESTED_TYPE, so we could
consider removing it from <type_traits>. I find it more
straightforward to use void_t directly.

	* include/std/functional (_Reference_wrapper_base_impl): Remove.
	(_Refwrap_base_arg1, _Refwrap_base_arg2): New helpers using __void_t.
	(_Reference_wrapper_base): Inherit from new helpers.

Tested powerpc64le-linux, committed to trunk.


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