Don't allow variables as attribute name

Attributes require a static class name...

This fixes https://oss-fuzz.com/testcase-detail/6267052359942144.

Author: nikic

Zend/tests/attributes/018_variable_attribute_name.phpt

@@ -0,0 +1,11 @@
+--TEST--
+Attribute name cannot be a variable
+--FILE--
+<?php
+
+<<$x>>
+class A {}
+
+?>
+--EXPECTF--
+Parse error: syntax error, unexpected '$x' (T_VARIABLE), expecting identifier (T_STRING) or static (T_STATIC) or namespace (T_NAMESPACE) or \\ (T_NS_SEPARATOR) in %s on line %d

Zend/zend_language_parser.y

@@ -325,11 +325,11 @@ attribute_arguments:
 ;
 
 attribute_decl:
-		class_name_reference
+		class_name
 			{ $$ = zend_ast_create(ZEND_AST_ATTRIBUTE, $1, NULL); }
-	|	class_name_reference '(' ')'
+	|	class_name '(' ')'
 			{ $$ = zend_ast_create(ZEND_AST_ATTRIBUTE, $1, NULL); }
-	|	class_name_reference '(' attribute_arguments ')'
+	|	class_name '(' attribute_arguments ')'
 			{ $$ = zend_ast_create(ZEND_AST_ATTRIBUTE, $1, $3); }
 ;