libstdc++: Fix ambiguous comparisons in __gnu_debug::bitset [PR 96303]

libstdc++: Fix ambiguous comparisons in __gnu_debug::bitset [PR 96303]

With -pedantic the debug mode bitset has an ambiguous equality
comparison operator, because it tries to compare the non-debug base to
the debug object. The base object can be converted to another debug
bitset, making the same operator== a candidate again.

The fix is to do the comparison on both base objects, so the operator
for the derived type isn't a candidate.

For the inequality operator the same change should be done, but that
operator can be removed entirely for C++20 because it can be synthesized
by the compiler.

I don't think either equality or inequality operators are really needed,
because the public _GLIBCXX_STD_C::bitset base class cam always be
compared using its own comparison operators. I'm not changing that here
though.

libstdc++-v3/ChangeLog:

	PR libstdc++/96303
	* include/debug/bitset (bitset::operator==): Call _M_base() on
	right operand.
	(bitset::operator!=): Likewise, but don't define it at all when
	default comparisons are supported by the compiler.
	* testsuite/23_containers/bitset/operations/96303.cc: New test.