Extract WritebackPseudoComponent back out as a superclass of UnpinPseudoComponent.  NFC.
Extract WritebackPseudoComponent back out as a superclass of
UnpinPseudoComponent.  NFC.

...this secondary use of LogicalLValueComponent for writebacks is
really a huge mess that I apologize for foisting on the world.
Exclusivity enforcement should eventually let us kill the awful
diagnoseWritebackConflict hack, which will make a simplification here
much easier.

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