[llvm] [TableGen] Use std::move instead of swap. NFC. (PR #81606)

[David Blaikie via llvm-commits]

dwblaikie wrote:

> Historically TableGen has used `A.swap(B)` to move containers without the expense of copying them. Perhaps this predated rvalue references. In any case `A = std::move(B)` seems like a more direct way to implement this when only A is required after the operation.

That's certainly a throwback indeed - nice cleanup!

https://github.com/llvm/llvm-project/pull/81606