These scopes would not exist in MIR and can cause ICEs with invalid uses
of let expressions.

There was an incomplete version of the check in parsing and a second
version in AST validation. This meant that some, but not all, invalid
uses were allowed inside macros/disabled cfgs. It also means that later
passes have a hard time knowing when the let expression is in a valid
location, sometimes causing ICEs.

- Add a field to ExprKind::Let in AST/HIR to mark whether it's in a
  valid location.
- Suppress later errors and MIR construction for invalid let
  expressions.

Previously some invalid let expressions would result in both a feature
error and a parsing error. Avoid this and ensure that we only emit the
parsing error when this happens.

- Add doc comment to new type
- Restore "only supported directly in conditions of `if` and `while` expressions" note
- Rename variant with clearer name