In this PR, we were unnecessarily rejecting a constexpr initializer_list
with no elements.  This seems like a fairly useless degenerate case, but it
makes sense to avoid allocating an underlying array at all if there are no
elements and instead use a null pointer, like the initializer_list default
constructor.

If the (automatic storage duration) list does have initializer elements, we
continue to reject the declaration, because the initializer_list ends up
referring to an automatic storage duration temporary array, which is not a
suitable constant initializer.  If we make it static, it should be OK
because we refer to a static array.  The second hunk fixes that case.  It
also means we won't diagnose some real errors in templates, but those
diagnostics aren't required, and we'll get them when the template is
instantiated.

	* call.c (convert_like_real) [ck_list]: Don't allocate a temporary
	array for an empty list.
	* typeck2.c (store_init_value): Don't use cxx_constant_init in a
	template.

From-SVN: r268827