Auto merge of #33989 - eddyb:mir-viz, r=nikomatsakis

[MIR] Make scopes debuginfo-specific (visibility scopes).

Fixes #32949 by having MIR (visibility) scopes mimic the lexical structure.
Unlike #33235, this PR also removes all scopes without variable bindings.

Printing of scopes also changed, e.g. for:
```rust
fn foo(x: i32, y: i32) { let a = 0; let b = 0; let c = 0; }
```
Before my changes:
```rust
fn foo(arg0: i32, arg1: i32) -> () {
    let var0: i32;                       // "x" in scope 1 at <anon>:1:8: 1:9
    let var1: i32;                       // "y" in scope 1 at <anon>:1:16: 1:17
    let var2: i32;                       // "a" in scope 3 at <anon>:1:30: 1:31
    let var3: i32;                       // "b" in scope 6 at <anon>:1:41: 1:42
    let var4: i32;                       // "c" in scope 9 at <anon>:1:52: 1:53

    ...

    scope tree:
    0 1 2 3 {
        4 5
        6 {
            7 8
            9 10 11
        }
    }
}
```
After my changes:
```rust
fn foo(arg0: i32, arg1: i32) -> () {
    scope 1 {
        let var0: i32;                   // "x" in scope 1 at <anon>:1:8: 1:9
        let var1: i32;                   // "y" in scope 1 at <anon>:1:16: 1:17
        scope 2 {
            let var2: i32;               // "a" in scope 2 at <anon>:1:30: 1:31
            scope 3 {
                let var3: i32;           // "b" in scope 3 at <anon>:1:41: 1:42
                scope 4 {
                    let var4: i32;       // "c" in scope 4 at <anon>:1:52: 1:53
                }
            }
        }
    }

    ...
}

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