[llvm] [ObjCopy] Use StringRef::starts_with (NFC) (PR #139408)

[Kazu Hirata via llvm-commits]

kazutakahirata wrote:

@MaskRay

> Can use consume_front

Yes, I've thought about, but I wasn't sure if the use of a temporary variable is elegant.  Thoughts?

```
      StringRef Tmp = Sym.Name;
      if (Tmp.consumeFront(Config.SymbolsPrefixRemove))
        Sym.Name = Tmp;
```


https://github.com/llvm/llvm-project/pull/139408