Recover on `const X = 42;` and infer type + Error Stash API
Here we:
1. Introduce a notion of the "error stash".
This is a map in the `Handler` to which you can `err.stash(...)` away your diagnostics and then steal them in a later "phase" of the compiler (e.g. stash in parser, steal in typeck) to enrich them with more information that isn't available in the previous "phase".
I believe I've covered all the bases to make sure these diagnostics are actually emitted eventually even under `#[cfg(FALSE)]` but please check my logic.
2. Recover when parsing `[const | static mut?] $ident = $expr;` which has a missing type.
Use the "error stash" to stash away the error and later steal the error in typeck where we emit the error as `MachineApplicable` with the actual inferred type. This builds on https://github.com/rust-lang/rust/pull/62804.
cc https://github.com/rust-lang/rfcs/pull/2545
r? @estebank
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