PR libstdc++/86751 default assignment operators for std::pair

PR libstdc++/86751 default assignment operators for std::pair

The solution for PR 77537 causes ambiguities due to the extra copy
assignment operator taking a __nonesuch_no_braces parameter. By making
the base class non-assignable we don't need the extra deleted overload
in std::pair. The copy assignment operator will be implicitly deleted
(and the move assignment operator not declared) as needed. Without the
additional user-provided operator in std::pair the ambiguity is avoided.

	PR libstdc++/86751
	* include/bits/stl_pair.h (__pair_base): New class with deleted copy
	assignment operator.
	(pair): Derive from __pair_base.
	(pair::operator=): Remove deleted overload.
	* python/libstdcxx/v6/printers.py (StdPairPrinter): New pretty printer
	so that new base class isn't shown in GDB.
	* testsuite/20_util/pair/86751.cc: New test.
	* testsuite/20_util/pair/ref_assign.cc: New test.

From-SVN: r263185