Unary `-` does not check for overflow

[simias]

The following code prints -2147483648 -2147483648:

fn main() {
    let i = 0x80000000u32 as i32;

    println!("{} {}", i, -i);
}

Both 0 - i and i / -1 generate an overflow error but it seems the unary - gladly wraps around.

[alexcrichton]

I added this to the list of unimplemented overflow checks on #22020, but thanks for the report!