libstdc++: Access std::pair members without tuple-like helpers

This avoids the tuple-like API for std::pair in the unordered
containers, removing some overly generic code.

The _Select1st projection can figure out the member types of a std::pair
without using decltype(std::get<0>(...)).

We don't need _Select2nd because it's only needed in
_NodeBuilder::_S_build, and that can just access the .second member of
the pair directly. The return type of that function doesn't need to be
deduced by decltype, we can just expose the __node_type typedef of the
node generator.

libstdc++-v3/ChangeLog:

	* include/bits/hashtable_policy.h (_Select1st): Replace use of
	std::get.
	(_Select2nd): Remove.
	(_NodeBuilder::_S_build): Use _NodeGenerator::__node_type
	typedef instead of deducing it. Remove unnecessary piecewise
	construction.
	(_ReuseOrAllocNode): Make __node_type public.
	(_Map_base): Adjust partial specialization to be able to extract
	the mapped_type without using tuple_element.
	(_Map_base::at): Define inline
	* testsuite/23_containers/unordered_map/requirements/53339.cc:
	Remove XFAIL.
	* testsuite/23_containers/unordered_multimap/requirements/53339.cc:
	Likewise.

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