PR libstdc++/87822 fix layout change for nested std::pair

The introduction of the empty __pair_base base class for PR 86751
changed the layout of std::pair<std::pair<...>, ...>. The outer pair and
its first member both have a base class of the same type, which cannot
exist at the same address. This causes the first member to be at a
non-zero offset.

The solution is to make the base class depend on the template
parameters, so that each pair type has a different base class type,
which allows the base classes of the outer pair and its first member to
have the same address.

	PR libstdc++/87822
	* include/bits/stl_pair.h (__pair_base): Change to class template.
	(pair): Make base class type depend on template parameters.
	* testsuite/20_util/pair/87822.cc: New test.

From-SVN: r265678