libstdc++: Implement P2255R2 dangling checks for std::tuple [PR108822]

This is the last part of PR libstdc++/108822 implementing P2255R2, which
makes it ill-formed to create a std::tuple that would bind a reference
to a temporary.

The dangling checks are implemented as deleted constructors for C++20
and higher, and as Debug Mode static assertions in the constructor body
for older standards. This is similar to the r13-6084-g916ce577ad109b
changes for std::pair.

As part of this change, I've reimplemented most of std::tuple for C++20,
making use of concepts to replace the enable_if constraints, and using
conditional explicit to avoid duplicating most constructors. We could
use conditional explicit for the C++11 implementation too (with pragmas
to disables the -Wc++17-extensions warnings), but that should be done as
a stage 1 change for GCC 15 rather than now.

The partial specialization for std::tuple<T1, T2> is no longer used for
C++20 (or more precisely, for a C++20 compiler that supports concepts
and conditional explicit). The additional constructors and assignment
operators that take std::pair arguments have been added to the C++20
implementation of the primary template, with sizeof...(_Elements)==2
constraints. This avoids reimplementing all the other constructors in
the std::tuple<T1, T2> partial specialization to use concepts. This way
we avoid four implementations of every constructor and only have three!
(The primary template has an implementation of each constructor for
C++11 and another for C++20, and the tuple<T1,T2> specialization has an
implementation of each for C++11, so that's three for each constructor.)

In order to make the constraints more efficient on the C++20 version of
the default constructor I've also added a variable template for the
__is_implicitly_default_constructible trait, implemented using concepts.

libstdc++-v3/ChangeLog:

	PR libstdc++/108822
	* include/std/tuple (tuple): Add checks for dangling references.
	Reimplement constraints and constant expressions using C++20
	features.
	* include/std/type_traits [C++20]
	(__is_implicitly_default_constructible_v): Define.
	(__is_implicitly_default_constructible): Use variable template.
	* testsuite/20_util/tuple/dangling_ref.cc: New test.

Reviewed-by: Patrick Palka <ppalka@redhat.com>