[1.11.x] Fixed #27915 -- Allowed Meta.indexes to be defined in abstract models.

Thanks Markus Holtermann for review.

Backport of 3d19d1428a from master

tests/model_indexes/models.py

@@ -5,3 +5,19 @@ class Book(models.Model):
     title = models.CharField(max_length=50)
     author = models.CharField(max_length=50)
     pages = models.IntegerField(db_column='page_count')
+
+
+class AbstractModel(models.Model):
+    name = models.CharField(max_length=50)
+
+    class Meta:
+        abstract = True
+        indexes = [models.indexes.Index(fields=['name'])]
+
+
+class ChildModel1(AbstractModel):
+    pass
+
+
+class ChildModel2(AbstractModel):
+    pass