cgraph: Add toplevel_node

cgraph: Add toplevel_node

asm_node and symbol_node will now inherit from toplevel_node.
This is now useful for lto partitioning, in future it should be also
useful for toplevel extended assembly.

gcc/ChangeLog:

	* cgraph.h (enum symtab_type): Replace with toplevel_type.
	(enum toplevel_type): New.
	(struct toplevel_node): New.
	(struct asm_node): Inherits from toplevel_node.
	(test): New.
	(symbol_table::finalize_toplevel_asm): Use constructor.
	* coretypes.h (struct toplevel_node): New.
	(struct asm_node): New.
	* lto-streamer-in.cc (lto_input_toplevel_asms): Add
	lto_file_data.