Associated functions that contain extern indicator or have `#[rustc_std_internal_symbol]` are reachable
Previously these fails to link with ``undefined reference to `foo'``:
<details>
<summary>Example 1</summary>
```rs
struct AssocFn;
impl AssocFn {
#[no_mangle]
fn foo() {}
}
fn main() {
extern "Rust" {
fn foo();
}
unsafe { foo() }
}
```
([Playground](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=f1244afcdd26e2a28445f6e82ca46b50))
</details>
<details>
<summary>Example 2</summary>
```rs
#![crate_name = "lib"]
#![crate_type = "lib"]
struct AssocFn;
impl AssocFn {
#[no_mangle]
fn foo() {}
}
```
```rs
extern crate lib;
fn main() {
extern "Rust" {
fn foo();
}
unsafe { foo() }
}
```
</details>
But I believe they should link successfully, because this works:
<details>
```rs
#[no_mangle]
fn foo() {}
fn main() {
extern "Rust" {
fn foo();
}
unsafe { foo() }
}
```
([Playground](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=789b3f283ee6126f53939429103ed98d))
</details>
This PR fixes the problem, by adding associated functions that have "custom linkage" to `reachable_set`, just like normal functions.
I haven't tested whether #76211 and [Miri](https://github.com/rust-lang/miri/issues/1837) are fixed by this PR yet, but I'm submitting this anyway since this fixes the examples above.
I added a `run-pass` test that combines my two examples above, but I'm not sure if that's the right way to test this. Maybe I should add / modify an existing codegen test (`src/test/codegen/export-no-mangle.rs`?) instead?
5a19ffe1c2