[IRLinker] make IRLinker::AddLazyFor optional (llvm::unique_function). NFC

2 of the 3 callsite of IRMover::move() pass empty lambda functions. Just
make this parameter llvm::unique_function.

Came about via discussion in D120781. Probably worth making this change
regardless of the resolution of D120781.

Reviewed By: dexonsmith

Differential Revision: https://reviews.llvm.org/D121630