Currently, when declaring an extern weak function in Rust, we use the
following syntax:
```rust
unsafe extern "C" {
#[linkage = "extern_weak"]
static FOO: Option<unsafe extern "C" fn() -> ()>;
}
```
This allows runtime-checking the extern weak symbol through the Option.
When emitting LLVM-IR, the Rust compiler currently emits this static
as an i8, and a pointer that is initialized with the value of the global
i8 and represents the nullabilty e.g.
```
@FOO = extern_weak global i8
@_rust_extern_with_linkage_FOO = internal global ptr @FOO
```
This approach does not work well with CFI, where we need to attach CFI
metadata to a concrete function declaration, which was pointed out in
https://github.com/rust-lang/rust/issues/115199.
This change switches to emitting a proper function declaration instead
of a global i8. This allows CFI to work for extern_weak functions.
We keep initializing the Rust internal symbol with the function
declaration, which preserves the correct behavior for runtime checking
the Option.
Co-authored-by: Jakob Koschel <jakobkoschel@google.com>
