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_ast_pretty/src/pprust/state/expr.rs

@@ -352,7 +352,7 @@ impl<'a> State<'a> {
                 self.end();
                 self.word(")");
             }
-            ast::ExprKind::Let(pat, scrutinee, _) => {
+            ast::ExprKind::Let(pat, scrutinee, _, _) => {
                 self.print_let(pat, scrutinee);
             }
             ast::ExprKind::If(test, blk, elseopt) => self.print_if(test, blk, elseopt.as_deref()),