Auto merge of #86246 - FabianWolff:issue-83471, r=estebank

Add basic checks for well-formedness of `fn`/`fn_mut` lang items

This pull request fixes #83471. Lang items are never actually checked for well-formedness (#9307). This means that one can get an ICE quite easily, e.g. as follows:
```rust
#![feature(lang_items)]
#[lang = "fn"]
trait MyFn {
    const call: i32 = 42;
}

fn main() {
    (|| 42)();
}
```
or this:
```rust
#![feature(lang_items)]
#[lang = "fn"]
trait MyFn {
    fn call(i: i32, j: i32);
}

fn main() {
    (|| 42)();
}
```
Ideally, there should probably be a more comprehensive strategy for checking lang items for well-formedness, but for the time being, I have added some rudimentary well-formedness checks that prevent #83471 and similar issues.

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