Use `absl::StrFormat()` in `Reg<T>::ToString()`
Use `absl::StrFormat()` in `Reg<T>::ToString()`

PiperOrigin-RevId: 322528126
Change-Id: Ia5344e53366a8b3c11ec0dbba7cff8e4192a7605

google/sandboxed-api,google/sandboxed-api,google/sandboxed-api,google/sandboxed-api