libstdc++: Implement P2255R2 dangling checks for std::pair

libstdc++: Implement P2255R2 dangling checks for std::pair

This uses the new __reference_constructs_from_temporary built-in to
identify when a std::pair constructor will bind a reference to a
temporary that goes out of scope at the end of the constructor.  For
example, std::pair<const long&, int> p(1, 2); will call the pair<const
long&, int>::pair(U1&&, U2&&) constructor with U1=int and U2=int. In the
constructor body a temporary long will be created and the p.first member
will bind to that temporary. When the constructor returns, the reference
is immediately dangling. P2255 requires the constructor to be deleted to
prevent this bug.

Although P2255 was approved for C++23, it fixes a longstanding LWG issue
in older standards, and it turns silent runtime undefined behaviour into
a compilation error. Because of that, the dangling checks are applied
all the way back to C++98.  However, if these changes cause too much
code to be rejected (e.g. in cases where the dangling reference is never
used after the constructor returns) then we can consider removing them
for C++20 and older standards.

The affected constructors are deleted for C++20 and later, when concepts
are available to simplify the constructor constraints. For C++17 and
earlier the overload sets are complicated and awkward to maintain, so
the dangling checks are done in static assertions in the constructor
bodies, instead of being SFINAE-friendly constraints. The pre-C++17
assertions are only enabled for Debug Mode, to avoid introducing a
breaking change in Stage 4. We should consider enabling them by default
in Stage 1 for GCC 14.

libstdc++-v3/ChangeLog:

	* include/bits/stl_pair.h (pair) [C++20]: Add non-dangling
	constraints to constructors and add deleted overloads for the
	dangling cases, as per P2255R2.
	(pair) [!C++20 && _GLIBCXX_DEBUG]: Add static assertions to
	make dangling cases ill-formed.
	* testsuite/20_util/pair/dangling_ref.cc: New test.