d: Factor aggregate_initializer_decl to set the sinit for aggregate declarations.

d: Factor aggregate_initializer_decl to set the sinit for aggregate declarations.

The self-hosted implementation of the D front-end changes the type of
`sinit' to a void pointer, which requires an explicit cast to `tree'.

gcc/d/ChangeLog:

	* decl.cc (DeclVisitor::visit (StructDeclaration *)): Don't use sinit
	for declaration directly.
	(DeclVisitor::visit (ClassDeclaration *)): Likewise.
	(aggregate_initializer_decl): Likewise.  Set sinit after creating.