c++: Fix verify_ctor_sanity ICE [PR96241]

c++: Fix verify_ctor_sanity ICE [PR96241]

The code added in r10-6437 caused us to create a CONSTRUCTOR when we're
{}-initializing an aggregate.  Then we pass this new CONSTRUCTOR down to
cxx_eval_constant_expression which, if the CONSTRUCTOR isn't TREE_CONSTANT
or reduced_constant_expression_p, calls cxx_eval_bare_aggregate.  In
this case the CONSTRUCTOR wasn't reduced_constant_expression_p because
for r_c_e_p a CONST_DECL isn't good enough so it returns false.  So we
go to cxx_eval_bare_aggregate where we crash, because ctx->ctor wasn't
set up properly.  So my fix is to do so.  Since we're value-initializing,
I'm not setting CONSTRUCTOR_NO_CLEARING.  To avoid keeping a garbage
constructor around, I call free_constructor in case the evaluation did
not use it.

gcc/cp/ChangeLog:

	PR c++/96241
	* constexpr.c (cxx_eval_array_reference): Set up ctx->ctor if we
	are initializing an aggregate.  Call free_constructor on the new
	CONSTRUCTOR if it isn't returned from cxx_eval_constant_expression.

gcc/testsuite/ChangeLog:

	PR c++/96241
	* g++.dg/cpp0x/constexpr-96241.C: New test.
	* g++.dg/cpp1y/constexpr-96241.C: New test.

(cherry picked from commit 0df73beea03f9dc124dc6e98ec4bdeacca7a2eea)