Virtual props xmlreader

[nielsdos]

Allow ZEND_ACC_VIRTUAL to be used to not have property backing storage without resorting to hooks

This is useful to reduce the memory usage of objects that don't actually
use the backing storage. Examples are XMLReader and DOM. When the
properties were added to the stubs, these objects became much much
bigger, which is a waste of memory.

Closes #11644.

Works towards #13988.

Use @virtual annotation in XMLReader

All properties of XMLReader are virtual and therefore don't need backing
storage.

[iluuu1994]

I looked through all the usages of ZEND_ACC_VIRTUAL. The only thing I found was a sub-optimal error message. I don't see any other issues.

diff --git a/ext/standard/var_unserializer.re b/ext/standard/var_unserializer.re
index 69dd3f84d3a..52fb9e304c5 100644
--- a/ext/standard/var_unserializer.re
+++ b/ext/standard/var_unserializer.re
@@ -561,7 +561,7 @@ static int is_property_visibility_changed(zend_class_entry *ce, zval *key)
 				return 1;
 			} else {
 				php_error_docref(NULL, E_WARNING,
-					"Cannot unserialize value for hooked property %s::$%s",
+					"Cannot unserialize value for virtual property %s::$%s",
 					ZSTR_VAL(existing_propinfo->ce->name), Z_STRVAL_P(key));
 				zval_ptr_dtor_str(key);
 				return -1;

[nielsdos]

@iluuu1994 Thanks, I rebased this and applied your patch too.

@ericmann @SakiTakamachi @NattyNarwhal I'm asking RM permission to merge this into master as this is a new internal feature (not user-facing). It adds the ability to use the virtual annotation in the stub generator to tell the engine to avoid generating backing storage for the property where the annotation is defined upon.

[NattyNarwhal]

Tested and it seems to work and seems low-impact. I trust your judgement here for a merge.