Make extern "rust-intrinsic" coerce to unsafe Rust functions

[brson]

In addition to #10725, this is one of the things blocking replacing things like cast::transmute with intrinsics. Although they are callable like other Rust functions they cannot be assigned to normal Rust functions.

@nikomatsakis Interested in what you think.

[brson]

@pcwalton would rather make it impossible to take the value of intrinsics, which makes them more different from rust functions.

[steveklabnik]

Triage: no change

[steveklabnik]

I was trying to come up with some code to reproduce this, and here's what I had:

pub fn main() {
    unsafe {
        let copy: unsafe extern "rust-intrinsic" fn(*const i32, *mut i32, usize) = std::intrinsics::copy;
        let assign: unsafe fn(*const i32, *mut i32, usize) = std::mem::transmute(copy);
    }
}

This currently ICEs, and I filed #34123 with a smaller reproduction.

[brson]

Closing in favor of #34123