PR c++/79592 - missing explanation of invalid constexpr.
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