Auto merge of #115677 - matthewjasper:let-expr-recovery, r=b-naber

Improve invalid let expression handling

- Move all of the checks for valid let expression positions to parsing.
- Add a field to ExprKind::Let in AST/HIR to mark whether it's in a valid location.
- Suppress some later errors and MIR construction for invalid let expressions.
- Fix a (drop) scope issue that was also responsible for #104172.

Fixes #104172
Fixes #104868

Author: bors