libstdc++: Heterogeneous std::pair comparisons [PR113386]

libstdc++: Heterogeneous std::pair comparisons [PR113386]

I'm only treating this as a DR for C++20 for now, because it's less work
and only requires changes to operator== and operator<=>. To do this for
older standards would require changes to the six relational operators
used pre-C++20.

libstdc++-v3/ChangeLog:

	PR libstdc++/113386
	* include/bits/stl_pair.h (operator==, operator<=>): Support
	heterogeneous comparisons, as per LWG 3865.
	* testsuite/20_util/pair/comparison_operators/lwg3865.cc: New
	test.