Rollup merge of #64698 - Centril:infer-const-with-stash, r=estebank

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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