Matthias Krüger 8f5c09b37c
Rollup merge of #138349 - 1c3t3a:external-weak-cfi, r=rcvalle
Emit function declarations for functions with `#[linkage="extern_weak"]`

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. Example:
```
`@_rust_extern_with_linkage_FOO` = internal global ptr `@FOO`
...
declare !type !61 !type !62 !type !63 !type !64 extern_weak void `@FOO(double)` unnamed_addr #6
```

We keep initializing the Rust internal symbol with the function declaration, which preserves the correct behavior for runtime checking the Option.

r? `@rcvalle`

cc `@jakos-sec`

try-job: test-various
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The files here use the LLVM FileCheck framework, documented at https://llvm.org/docs/CommandGuide/FileCheck.html.

One extension worth noting is the use of revisions as custom prefixes for FileCheck. If your codegen test has different behavior based on the chosen target or different compiler flags that you want to exercise, you can use a revisions annotation, like so:

// revisions: aaa bbb
// [bbb] compile-flags: --flags-for-bbb

After specifying those variations, you can write different expected, or explicitly unexpected output by using <prefix>-SAME: and <prefix>-NOT:, like so:

// CHECK: expected code
// aaa-SAME: emitted-only-for-aaa
// aaa-NOT:                        emitted-only-for-bbb
// bbb-NOT:  emitted-only-for-aaa
// bbb-SAME:                       emitted-only-for-bbb