Convert symtab, cgraph and varpool nodes into a real class hierarchy

This is the handwritten part of the patch; automated part to follow.

	* cgraph.h (symtab_node_base): Convert to a class;
	add GTY((desc ("%h.type"), tag ("SYMTAB_SYMBOL"))), and take
	chain_next/prev from symtab_node_def.
	(cgraph_node): Inherit from symtab_node; add GTY option
	tag ("SYMTAB_FUNCTION").
	(varpool_node): Inherit from symtab_node; add GTY option
	tag ("SYMTAB_VARIABLE").
	(symtab_node_def): Remove.
	(is_a_helper <cgraph_node>::test (symtab_node_def *)): Convert to...
	(is_a_helper <cgraph_node>::test (symtab_node_base *)): ...this.
	(is_a_helper <varpool_node>::test (symtab_node_def *)): Convert to...
	(is_a_helper <varpool_node>::test (symtab_node_base *)): ...this.

	* ipa-ref.h (symtab_node_def): Drop.
	(symtab_node): Change underlying type from symtab_node_def to
	symtab_node_base.
	(const_symtab_node): Likwise.

	* is-a.h: Update examples in comment.

	* symtab.c (symtab_hash): Change symtab_node_def to symtab_node_base.
	(assembler_name_hash): Likewise.

From-SVN: r204170