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.