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

[Jay Foad via llvm-commits]

jayfoad wrote:

The motivation is just to write the code in a more obvious way. As a secondary effect this could _conceivably_ reduce peak memory usage, since the memory for B could be freed slightly earlier; but I am not expecting that to be measurable.

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