We changed months back to use the pre-generic form for constexpr evaluation,
but explain_invalid_constexpr_fn was still using DECL_SAVED_TREE.  This
mostly works, but misses some issues due to folding.  So with this patch we
save the pre-generic form of constexpr functions even when we know they
can't produce a constant result.

	* constexpr.c (register_constexpr_fundef): Do store the body of a
	template instantiation that is not potentially constant.
	(explain_invalid_constexpr_fn): Look it up.
	(cxx_eval_call_expression): Check fundef->result.

From-SVN: r279473