Support overflowing_add in libcore

[philberty]

I tried this code:

mod intrinsics {
    extern "rust-intrinsic" {
        pub fn add_with_overflow<T>(x: T, y: T) -> (T, bool);
    }
}

pub enum Option<T> {
    None,
    Some(T),
}

#[lang = "i32"]
impl i32 {
    pub fn checked_add(self, rhs: Self) -> Option<Self> {
        let (a, b) = self.overflowing_add(rhs);
        if b {
            Option::None
        } else {
            Option::Some(a)
        }
    }

    pub fn overflowing_add(self, rhs: Self) -> (Self, bool) {
        let (a, b) = unsafe { intrinsics::add_with_overflow(self as i32, rhs as i32) };
        (a as Self, b)
    }
}

I expected to see this happen: compile without error

• unknown lang item i32 #1896
• Bad cast error when behind generics #1897

test.rs:24:31: error: unknown builtin intrinsic: add_with_overflow
   24 |         let (a, b) = unsafe { intrinsics::add_with_overflow(self as i32, rhs as i32) };
      |                               ^~~~~~~~~~

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• What version of Rust GCC were you using, git sha if possible.