libstdc++: Implement P1494 and P3641 Partial program correctness [PR119060]

libstdc++: Implement P1494 and P3641 Partial program correctness [PR119060]

This implements the library parts of P1494 as amended by P3641.  For GCC the
compiler itself treats stdio operations as equivalent to the observable
checkpoint and thus it does not appear to be necessary to add calls to those
functions (it will not alter the outcome).

This adds the facility for C++26, although there is no reason, in principle,
that it would not work back to C++11 at least.

	PR c++/119060

libstdc++-v3/ChangeLog:

	* include/bits/version.def: Add observable_checkpoint at present
	allowed from C++26.
	* include/bits/version.h: Regenerate.
	* include/std/utility: Add std::observable_checkpoint().
	* src/c++23/std.cc.in: Add obervable_checkpoint () to utility.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>