[cfe-dev] What is the advantage of declaring a function (again) LIBBUILTIN ?

[Nat! via cfe-dev]

In Builtins.def I see quite a few functions, which seem to "just" 
duplicate what is supposed to be in a header anyway.

An example is "NSLogv":

```
// void NSLogv(NSString *fmt, va_list args)
LIBBUILTIN(NSLogv, "vGa", "fP:0:", "Foundation/NSObjCRuntime.h", OBJC_LANG)
```

and it's declared in Foundation/NSObjCRuntime as

```
FOUNDATION_EXPORT void NSLogv(NSString *format, va_list args) 
NS_FORMAT_FUNCTION(1,0);
```

Just the same. I don't get the benefit by this duplication.

Ciao
    Nat!