c++: clarify reshape_init designator handling

While looking at r17-3010, I found the handling of designators in
reshape_init* unclear, particularly in reshape_init_r.  Jakub's patch
recurses directly from reshape_init_class to itself to avoid this confusion.

With this patch, if we see a designator in reshape_init_r, it designates a
member of TYPE, which means brace elision if !first_initializer_p.  For this
to be true I needed to adjust reshape_init_array_1 to use
reshape_single_init from c++/103337.  And then if we see a designator and
TYPE is a class or array, we know we're initializing an element so we don't
consider initializing TYPE directly.

gcc/cp/ChangeLog:

	* decl.cc (reshape_single_init): Declare sooner.
	(reshape_init_array_1): Use it.
	(reshape_init_class): Drop "subclass" case.
	(reshape_init_r): d->cur->index always means initializing
	an element.