Detect missing if let or let-else

During `let` binding parse error and encountering a block, detect if there is a likely missing `if` or `else`:

```
error: expected one of `.`, `;`, `?`, `else`, or an operator, found `{`
  --> $DIR/missing-if-let-or-let-else.rs:14:25
   |
LL |     let Some(x) = foo() {
   |                         ^ expected one of `.`, `;`, `?`, `else`, or an operator
   |
help: you might have meant to use `if let`
   |
LL |     if let Some(x) = foo() {
   |     ++
help: alternatively, you might have meant to use `let else`
   |
LL |     let Some(x) = foo() else {
   |                         ++++
```

compiler/rustc_parse/src/lib.rs

@@ -6,6 +6,7 @@
 #![feature(assert_matches)]
 #![feature(box_patterns)]
 #![feature(debug_closure_helpers)]
+#![feature(default_field_values)]
 #![feature(if_let_guard)]
 #![feature(iter_intersperse)]
 #![recursion_limit = "256"]