Fix bug preventing parsing attributes on AST_CLASS

[TysonAndre]

Unexpectedly, the attributes were found at index 4, not index 3.
Index 3 was always null.
Use a different name to make it obvious if refactoring bugs cause the
placeholder index to have child nodes get unintentionally emitted.

Tests pass locally in 8.0, and in 7.0-7.4 in travis

[nikic]

Hm, I'm thinking it would make more sense to change the structure in php-src. All the function types have consistent AST structure, but there's no reason at all why classes should be consistent with functions.

[TysonAndre]

Hm, I'm thinking it would make more sense to change the structure in php-src. All the function types have consistent AST structure, but there's no reason at all why classes should be consistent with functions.

That's probably also fine with me, extensions that use the ast output other than this should be rare.
I didn't know if there were other reasons, and didn't know if there'd be objections to changing the internal structure after the feature freeze

[TysonAndre]

It doesn't actually save any memory, apparently, but is easy enough to move to position 3

ZEND_API zend_ast *zend_ast_create_decl(
	zend_ast_kind kind, uint32_t flags, uint32_t start_lineno, zend_string *doc_comment,
	zend_string *name, zend_ast *child0, zend_ast *child1, zend_ast *child2, zend_ast *child3, zend_ast *child4
) {
	zend_ast_decl *ast;

	ast = zend_ast_alloc(sizeof(zend_ast_decl));
	ast->kind = kind;
	ast->attr = 0;
	ast->start_lineno = start_lineno;
	ast->end_lineno = CG(zend_lineno);
	ast->flags = flags;
	ast->lex_pos = LANG_SCNG(yy_text);
	ast->doc_comment = doc_comment;
	ast->name = name;
	ast->child[0] = child0;
	ast->child[1] = child1;
	ast->child[2] = child2;
	ast->child[3] = child3;
	ast->child[4] = child4;

	return (zend_ast *) ast;
}

/* Separate structure for function and class declaration, as they need extra information. */
typedef struct _zend_ast_decl {
	zend_ast_kind kind;
	zend_ast_attr attr; /* Unused - for structure compatibility */
	uint32_t start_lineno;
	uint32_t end_lineno;
	uint32_t flags;
	unsigned char *lex_pos;
	zend_string *doc_comment;
	zend_string *name;
	zend_ast *child[5];
} zend_ast_decl;