[llvm] [BOLT] Gadget scanner: clarify MCPlusBuilder callbacks interface (PR #136147)

[Anatoly Trosinenko via llvm-commits]

atrosinenko wrote:

Added another tiny simplification related to this PR that was initially planned for a follow-up refactoring - simplified `traceReg` function in 4728a4748934da93f8a65464ae234c566ca02ba7: I kept printing "(none)" for `NoRegister` to keep the function generic, but printing "(error)" is not relevant anymore, as `ErrorOr<MCPhysReg>` is not used.

https://github.com/llvm/llvm-project/pull/136147