c++: indirect change of active union member in constexpr [PR101631,PR102286]

This patch adds checks for attempting to change the active member of a
union by methods other than a member access expression.

To be able to properly distinguish `*(&u.a) = ` from `u.a = `, this
patch redoes the solution for c++/59950 to avoid extranneous *&; it
seems that the only case that needed the workaround was when copying
empty classes.

This patch also ensures that constructors for a union field mark that
field as the active member before entering the call itself; this ensures
that modifications of the field within the constructor's body don't
cause false positives (as these will not appear to be member access
expressions). This means that we no longer need to start the lifetime of
empty union members after the constructor body completes.

As a drive-by fix, this patch also ensures that value-initialised unions
are considered to have activated their initial member for the purpose of
checking stores and accesses, which catches some additional mistakes
pre-C++20.

	PR c++/101631
	PR c++/102286

gcc/cp/ChangeLog:

	* call.cc (build_over_call): Fold more indirect refs for trivial
	assignment op.
	* class.cc (type_has_non_deleted_trivial_default_ctor): Create.
	* constexpr.cc (cxx_eval_call_expression): Start lifetime of
	union member before entering constructor.
	(cxx_eval_component_reference): Check against first member of
	value-initialised union.
	(cxx_eval_store_expression): Activate member for
	value-initialised union. Check for accessing inactive union
	member indirectly.
	* cp-tree.h (type_has_non_deleted_trivial_default_ctor):
	Forward declare.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/constexpr-89336-3.C: Fix union initialisation.
	* g++.dg/cpp1y/constexpr-union6.C: New test.
	* g++.dg/cpp1y/constexpr-union7.C: New test.
	* g++.dg/cpp2a/constexpr-union2.C: New test.
	* g++.dg/cpp2a/constexpr-union3.C: New test.
	* g++.dg/cpp2a/constexpr-union4.C: New test.
	* g++.dg/cpp2a/constexpr-union5.C: New test.
	* g++.dg/cpp2a/constexpr-union6.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
Reviewed-by: Jason Merrill <jason@redhat.com>