Auto merge of #56231 - eddyb:mir-debuginfo, r=oli-obk

rustc: move debug info from LocalDecl and UpvarDecl into a dedicated VarDebugInfo.

This PR introduces a MIR "user variable" debuginfo system, which amounts to mapping a variable name, in some `SourceScope`, to a `Place`, so that:

* each name can appear multiple times (e.g. due to macro hygiene), even in the same scope
* each `Place` can appear multiple times (e.g. in the future from optimizations like NRVO, which collapse multiple MIR locals into one)
* the `Place`s aren't limited to just locals, so they can describe the (right now quite ad-hoc) closure upvars and generator saved state fields, and can be properly transformed by optimizations (e.g. inlining - see `src/test/mir-opt/inline-closure-captures.rs`)

The main motivation for this was that #48300 and further optimizations were blocked on being able to describe complex debuginfo transformations (see https://github.com/rust-lang/rust/pull/48300#discussion_r170020762).

<hr/>

In the textual representation, the "user variable" debuginfo can be found in each scope, and consists of `debug NAME => PLACE;` "declarations", e.g. the MIR for `let x = ...; let y = ...; ...` is now:
```rust
    let _1: T;                           // in scope 0 at ...
    scope 1 {
        debug x => _1;                   // in scope 1 at ...
        let _2: T;                       // in scope 1 at ...
        scope 2 {
            debug y => _2;               // in scope 2 at ...
        }
    }
```
For reference, this is how the information was represented before this PR:
(notably, the scopes in which the variables are visible for debuginfo weren't even shown anywhere, making `scope 2` look pointless, and user variable names were part of MIR locals)
```rust
    let _1: T;                           // "x" in scope 0 at ...
    scope 1 {
        let _2: T;                       // "y" in scope 1 at ...
        scope 2 {
        }
    }
```

cc @nikomatsakis @michaelwoerister

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