Move let expression checking to parsing

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.

compiler/rustc_builtin_macros/src/assert/context.rs

@@ -241,7 +241,7 @@ impl<'cx, 'a> Context<'cx, 'a> {
                 self.manage_cond_expr(prefix);
                 self.manage_cond_expr(suffix);
             }
-            ExprKind::Let(_, local_expr, _) => {
+            ExprKind::Let(_, local_expr, _, _) => {
                 self.manage_cond_expr(local_expr);
             }
             ExprKind::Match(local_expr, _) => {