Fix a fundamental problem with TypeBuilder.handle by deleting it

TypeBuilder.handle creates an immutable_type gets the handle and then deletes the object to which the handle belonged to.
This was fundamentally a UAF. immutable_copy is an error prone api as its very easy to misuse. We should consider a re-architecture of this API. `TypeBuilder.immutable_copy().<anything>` or `Type.mutable_copy().<anything>` should be considered sketchy and immutable_copy().handle is just a straight up UAF. Prior to this commit the following would cause a crash:

current_data_variable.type = PointerBuilder.create(FunctionType.create())

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