c++: constexpr union placement new [PR121068]HEADtrunkmaster

The note and example in [class.union] p6 think that placement new can be
used to change the active member of a union, but we didn't support that for
array members in constant-evaluation even after implementing P1330 and
P2747.

First I tried to address this by introducing a CLOBBER_BEGIN_OBJECT for the
entire array, but that broke the resolution of LWG3436, which invokes 'new
T[1]' for an array T, and trying to clobber a multidimensional array when
the actual object is single-dimensional breaks.  So I've raised that issue
with the committee.  Until that is resolved, this patch takes a simpler
approach: allow initialization of an element of an array to make the array
the active member of a union.

	PR c++/121068

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_eval_store_expression): Allow ARRAY_REFs
	when activating an array member of a union.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/constexpr-union6.C: Expect x5 to work.
	* g++.dg/cpp26/constexpr-new4.C: New test.

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