* include/bits/stl_pair.h (pair): Add _GLIBCXX20_CONSTEXPR to
	piecewise construction constructor, assignment operators, and swap.
	* include/std/tuple (pair::pair(piecewise_construct_t, tuple, tuple)):
	Add _GLIBCXX20_CONSTEXPR.
	(pair::pair(tuple, tuple, _Index_tuple, _Index_tuple)): Likewise.
	* testsuite/20_util/pair/constexpr_assign.cc: New test.
	* testsuite/20_util/pair/constexpr_swap.cc: New test.

From-SVN: r276154