PR c++/91369 - constexpr destructor and member initializer.

PR c++/91369 - constexpr destructor and member initializer.

Previously it didn't matter whether we looked through a TARGET_EXPR in
constexpr evaluation, but now that we have constexpr destructors it does.
On IRC I mentioned the idea of clearing TARGET_EXPR_CLEANUP in
digest_nsdmi_init, but since this initialization is expressed by an
INIT_EXPR, it's better to handle all INIT_EXPR, not just those for a member
initializer.

	* constexpr.c (cxx_eval_store_expression): Look through TARGET_EXPR
	when not preevaluating.

From-SVN: r280018