Fixed #32632, Fixed #32657 -- Removed flawed support for Subquery deconstruction.

Subquery deconstruction support required implementing complex and
expensive equality rules for sql.Query objects for little benefit as
the latter cannot themselves be made deconstructible to their reference
to model classes.

Making Expression @deconstructible and not BaseExpression allows
interested parties to conform to the "expression" API even if they are
not deconstructible as it's only a requirement for expressions allowed
in Model fields and meta options (e.g. constraints, indexes).

Thanks Phillip Cutter for the report.

This also fixes a performance regression in bbf141bcdc.

django/db/models/query_utils.py

@@ -5,6 +5,7 @@ Factored out from django.db.models.query to avoid making the main module very
 large and/or so that they can be used by other modules without getting into
 circular import difficulties.
 """
+import copy
 import functools
 import inspect
 from collections import namedtuple
@@ -43,14 +44,11 @@ class Q(tree.Node):
         if not(isinstance(other, Q) or getattr(other, 'conditional', False) is True):
             raise TypeError(other)
 
-        # If the other Q() is empty, ignore it and just use `self`.
-        if not other:
+        if not self:
+            return other.copy() if hasattr(other, 'copy') else copy.copy(other)
+        elif isinstance(other, Q) and not other:
             _, args, kwargs = self.deconstruct()
             return type(self)(*args, **kwargs)
-        # Or if this Q is empty, ignore it and just use `other`.
-        elif not self:
-            _, args, kwargs = other.deconstruct()
-            return type(other)(*args, **kwargs)
 
         obj = type(self)()
         obj.connector = conn