libstdc++: Fix ambiguous std::pair constructors [PR101124]

libstdc++: Fix ambiguous std::pair constructors [PR101124]

The deprecated non-standard std::pair constructors that allow
constructing std::pair<move-only-type, pointer-type> from an rvalue and
a literal zero where not sufficiently constrained. They were viable when
constructing std::pair<copyable-type, pointer-type>, and that case
should work fine using the standard constructors.

Replace the constraints on the non-standard constructors so they are
only viable in cases that should actually be ill-formed according to the
standard.

Also rename __null_ptr_constant to __zero_as_null_pointer_constant so it
matches the name of the -Wzero-as-null-pointer-constant warning. Also
make the text of the deprecated warning describe the problem in more
detail.

libstdc++-v3/ChangeLog:

	PR libstdc++/101124
	* include/bits/stl_pair.h (pair): Adjust constraints on
	deprecated constructors accepting literal zero as null pointer
	constant. Improve wording of deprecated attribute.
	* testsuite/20_util/pair/cons/99957.cc: Check that deprecated
	constructors do not cause ambiguities for copyable types.