Because of forward/recursive references, the TYPE_SIZE, TYPE_ALIGN, and
TYPE_MODE of structs and enums were set before laying out its members.

This adds a new macro TYPE_FORWARD_REFERENCES for storing those forward
references against the incomplete type, laying them out after the type
has been completed.  Construction of the TYPE_DECL has also been moved
on earlier in the type generation pass, which will allow the possibility
of adding gdc-specific type attributes to the D front-end in the future.

gcc/d/ChangeLog:

	* d-attribs.cc (apply_user_attributes): Set ATTR_FLAG_TYPE_IN_PLACE
	only on incomplete types.
	* d-codegen.cc (copy_aggregate_type): Set TYPE_STUB_DECL after copy.
	* d-compiler.cc (Compiler::onParseModule): Adjust.
	* d-tree.h (AGGREGATE_OR_ENUM_TYPE_CHECK): Define.
	(TYPE_FORWARD_REFERENCES): Define.
	* decl.cc (gcc_attribute_p): Update documentation.
	(DeclVisitor::visit (StructDeclaration *)): Exit before building type
	node if gcc.attributes symbol.
	(DeclVisitor::visit (ClassDeclaration *)): Build type node and add
	TYPE_NAME to current binding level before emitting anything else.
	(DeclVisitor::visit (InterfaceDeclaration *)): Likewise.
	(DeclVisitor::visit (EnumDeclaration *)): Likewise.
	(build_type_decl): Move rest_of_decl_compilation() call to
	finish_aggregate_type().
	* types.cc (insert_aggregate_field): Move layout_decl() call to
	finish_aggregate_type().
	(insert_aggregate_bitfield): Likewise.
	(layout_aggregate_members): Adjust.
	(finish_incomplete_fields): New function.
	(finish_aggregate_type): Handle forward referenced field types.  Call
	rest_of_type_compilation() after completing the aggregate.
	(TypeVisitor::visit (TypeEnum *)): Don't set size and alignment until
	after apply_user_attributes().  Call rest_of_type_compilation() after
	completing the enumeral.
	(TypeVisitor::visit (TypeStruct *)): Call build_type_decl() before
	apply_user_attributes().  Don't set size, alignment, and mode until
	after apply_user_attributes().
	(TypeVisitor::visit (TypeClass *)): Call build_type_decl() before
	applly_user_attributes().