c++: Track lifetimes in constant evaluation [PR70331,PR96630,PR98675]

This adds rudimentary lifetime tracking in C++ constexpr contexts,
allowing the compiler to report errors with using values after their
backing has gone out of scope. We don't yet handle other ways of
accessing values outside their lifetime (e.g. following explicit
destructor calls).

	PR c++/96630
	PR c++/98675
	PR c++/70331

gcc/cp/ChangeLog:

	* constexpr.cc (constexpr_global_ctx::is_outside_lifetime): New
	function.
	(constexpr_global_ctx::get_value): Don't return expired values.
	(constexpr_global_ctx::get_value_ptr): Likewise.
	(constexpr_global_ctx::remove_value): Mark value outside
	lifetime.
	(outside_lifetime_error): New function.
	(cxx_eval_call_expression): No longer track save_exprs.
	(cxx_eval_loop_expr): Likewise.
	(cxx_eval_constant_expression): Add checks for outside lifetime
	values. Remove local variables at end of bind exprs, and
	temporaries after cleanup points.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/constexpr-lifetime1.C: New test.
	* g++.dg/cpp1y/constexpr-lifetime2.C: New test.
	* g++.dg/cpp1y/constexpr-lifetime3.C: New test.
	* g++.dg/cpp1y/constexpr-lifetime4.C: New test.
	* g++.dg/cpp1y/constexpr-lifetime5.C: New test.
	* g++.dg/cpp1y/constexpr-lifetime6.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>

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