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_resolve/src/late.rs

@@ -4176,7 +4176,7 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
                 self.resolve_expr(e, Some(&expr));
             }
 
-            ExprKind::Let(ref pat, ref scrutinee, _) => {
+            ExprKind::Let(ref pat, ref scrutinee, _, _) => {
                 self.visit_expr(scrutinee);
                 self.resolve_pattern_top(pat, PatternSource::Let);
             }