Handle field initialization using force-move assignment

[turbolent]

Closes #739

Description

Assignment using the force-move operator gets the current value, and if it is non-nil, aborts the program.

However, when the force-move operator is used in an initializer to initialize a field, getting the value will naturally result in no value, which is not an implementation error like in all other cases (e.g. in a non-initializer like a function).

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[joshuahannan]

Thank you for fixing!