Auto merge of #31656 - jonas-schievink:mirdump, r=nrc

This allows obtaining a textual MIR dump for individual items or all items in the crate.

I haven't added any tests since ~~I'm too lazy~~ this is an unstable debugging option, but I'll add one if required.

MIR for a single function can now be dumped using `rustc -Zunstable-options --unpretty mir=my_function` and no longer requires the use of in-source `#[rustc_mir]` attributes.

Blocks rust-lang/rust-playpen#154 (if MIR dump support from the playpen is even wanted).

Example output:
```rust
fn main() {
    let x = Some(0);
    x.unwrap_or_else(|| 1);
}
```

```
MIR for expr || 1 (id=16)
fn(arg0: [closure@test.rs:3:22: 3:26]) -> i32 {
    let mut tmp0: ();

    bb0: {
        return = const 1;
        goto -> bb1;
    }

    bb1: {
        return;
    }
}
MIR for fn main::main (id=4)
fn() -> () {
    let var0: core::option::Option<i32>; // x
    let mut tmp0: ();
    let mut tmp1: i32;
    let mut tmp2: core::option::Option<i32>;
    let mut tmp3: [closure@test.rs:3:22: 3:26];

    bb0: {
        var0 = core::option::Option::Some(const 0);
        tmp2 = var0;
        tmp3 = [closure@test.rs:3:22: 3:26];
        tmp1 = core::option::Option<T>::unwrap_or_else(tmp2, tmp3) -> bb2;
    }

    bb1: {
        return;
    }

    bb2: {
        drop(tmp1) -> bb3;
    }

    bb3: {
        return = ();
        goto -> bb1;
    }
}
```

0 files changed, 0 insertions, 0 deletions