Auto merge of #104975 - JakobDegen:custom_mir_let, r=oli-obk

`#![custom_mir]`: Various improvements

This PR makes a bunch of improvements to `#![custom_mir]`. Ideally this would be 4 PRs, one for each commit, but those would take forever to get merged and be a pain to juggle. Should still be reviewed one commit at a time though.

### Commit 1: Support arbitrary `let`

Before this change, all locals used in the body need to be declared at the top of the `mir!` invocation, which is rather annoying. We attempt to change that.

Unfortunately, we still have the requirement that the output of the `mir!` macro must resolve, typecheck, etc. Because of that, we can't just accept this in the THIR -> MIR parser because something like
```rust
{
    let x = 0;
    Goto(other)
}
other = {
    RET = x;
    Return()
}
```
will fail to resolve. Instead, the implementation does macro shenanigans to find the let declarations and extract them as part of the `mir!` macro. That *works*, but it is fairly complicated and degrades debuginfo by quite a bit. Specifically, the spans for any statements and declarations that are affected by this are completely wrong. My guess is that this is a net improvement though.

One way to recover some of the debuginfo would be to not support type annotations in the `let` statements, which would allow us to parse like `let $stmt:stmt`. That seems quite surprising though.

### Commit 2: Parse consts

Reuses most of the const parsing from regular Mir building for building custom mir

### Commit 3: Parse statics

Statics are slightly weird because the Mir primitive associated with them is a reference/pointer to them, so this is factored out separately.

### Commit 4: Fix some spans

A bunch of the spans were non-ideal, so we adjust them to be much more helpful.

r? `@oli-obk`

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