[mypyc] Clear only subtype refs in tp_dealloc of built-in subtypes

[p-sawicki]

Fixes #21859

Deallocating an instance of a class that inherits from a built-in type might fail an assertion because when we clear references during the deallocation, we call the generated tp_clear function that in turn calls tp_clear of the base class.

The functions put in the tp_clear slot of built-in types require that the instance is still alive (references > 0) which is not the case during deallocation.

To avoid this, split the tp_clear function for types with built-in bases into two: first (subtype_clear) clears only the references owned by the subtype, second (clear) does the same plus calls base type tp_clear.

clear is put into the tp_clear slot while subtype_clear can be called from tp_dealloc. tp_dealloc also calls base type tp_dealloc which clears the references owned by the base type, so the base type tp_clear call was not necessary.

For classes without a builtin base we continue to clear all their references in a single function.