c++: End lifetime of objects in constexpr after destructor call [PR71093]

This patch adds checks for using objects after they've been manually
destroyed via explicit destructor call. Currently this is only
implemented for 'top-level' objects; FIELD_DECLs and individual elements
of arrays will need a lot more work to track correctly and are left for
a future patch.

The other limitation is that destruction of parameter objects is checked
too 'early', happening at the end of the function call rather than the
end of the owning full-expression as they should be for consistency;
see cpp2a/constexpr-lifetime2.C. This is because I wasn't able to find a
good way to link the constructed parameter declarations with the
variable declarations that are actually destroyed later on to propagate
their lifetime status, so I'm leaving this for a later patch.

	PR c++/71093

gcc/cp/ChangeLog:

	* constexpr.cc (constexpr_global_ctx::get_value_ptr): Don't
	return NULL_TREE for objects we're initializing.
	(constexpr_global_ctx::destroy_value): Rename from remove_value.
	Only mark real variables as outside lifetime.
	(constexpr_global_ctx::clear_value): New function.
	(destroy_value_checked): New function.
	(cxx_eval_call_expression): Defer complaining about non-constant
	arg0 for operator delete. Use remove_value_safe.
	(cxx_fold_indirect_ref_1): Handle conversion to 'as base' type.
	(outside_lifetime_error): Include name of object we're
	accessing.
	(cxx_eval_store_expression): Handle clobbers. Improve error
	messages.
	(cxx_eval_constant_expression): Use remove_value_safe. Clear
	bind variables before entering body.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/constexpr-lifetime1.C: Improve error message.
	* g++.dg/cpp1y/constexpr-lifetime2.C: Likewise.
	* g++.dg/cpp1y/constexpr-lifetime3.C: Likewise.
	* g++.dg/cpp1y/constexpr-lifetime4.C: Likewise.
	* g++.dg/cpp2a/bitfield2.C: Likewise.
	* g++.dg/cpp2a/constexpr-new3.C: Likewise. New check.
	* g++.dg/cpp1y/constexpr-lifetime7.C: New test.
	* g++.dg/cpp2a/constexpr-lifetime1.C: New test.
	* g++.dg/cpp2a/constexpr-lifetime2.C: New test.

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

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