[rust-lang/rust] Unify labels pointing at the same span in move errors when possible

### ROOT CAUSE The issue arises because multiple error messages reference the same span but are presented as separate labels. This results in redundant or split labels in the error output, making the message less clear. The root cause is the lack of a mechanism to unify these labels during error reporting. ### CODE FIX ```rust // No code fix provided in the issue description or research context. // The issue requires unifying labels in error messages, but the necessary code changes are not detailed. // Therefore, the issue cannot be fixed without further context or code access. ```