c++: Fix ICE from verify_ctor_sanity [PR98295]

c++: Fix ICE from verify_ctor_sanity [PR98295]

In this testcase we're crashing during constexpr evaluation of the
ARRAY_REF b[0] as part of evaluation of the lambda's by-copy capture of b
(which is encoded as a VEC_INIT_EXPR<b>).  Since A's constexpr default
constructor is not yet defined, b's initialization is not actually
constant, but because A is an empty type, evaluation of b from
cxx_eval_array_ref is successful and yields an empty CONSTRUCTOR.
And since this CONSTRUCTOR is empty, we {}-initialize the desired array
element, and end up crashing from verify_ctor_sanity during evaluation
of this initializer because we updated new_ctx.ctor without updating
new_ctx.object: the former now has type A[3] and the latter is still the
target of a TARGET_EXPR for b[0][0] created from cxx_eval_vec_init
(and so has type A).

This patch fixes this by setting new_ctx.object appropriately at the
same time that we set new_ctx.ctor from cxx_eval_array_reference.

gcc/cp/ChangeLog:

	PR c++/98295
	* constexpr.c (cxx_eval_array_reference): Also set
	new_ctx.object when setting new_ctx.ctor.

gcc/testsuite/ChangeLog:

	PR c++/98295
	* g++.dg/cpp0x/constexpr-98295.C: New test.