Remove Session.if_let_suggestions

We can instead if either the LHS or RHS types contain
`TyKind::Error`. In addition to covering the case where
we would have previously updated `if_let_suggestions`, this might
also prevent redundant errors in other cases as well.

compiler/rustc_resolve/src/late/diagnostics.rs

@@ -215,7 +215,6 @@ impl<'a: 'ast, 'ast> LateResolutionVisitor<'a, '_, 'ast> {
                     "let ".to_string(),
                     Applicability::MaybeIncorrect,
                 );
-                self.r.session.if_let_suggestions.borrow_mut().insert(*span);
             }
             _ => {}
         }