Support for let declarations. (#3257)

A `let` declaration is represented by a `bind_name` node in SemIR:

```carbon-semir
  %b: i32 = bind_name "b", %a
```

Because `Check` encounters the pattern before it sees the value, we
first create the `bind_name` node with an unset value and don't add it
to the block. Then, once we've seen and converted the initializer, we
update the `bind_name` to have the value and add it to the current
block, after the initializer code.

toolchain/parse/handle_statement.cpp

@@ -30,6 +30,10 @@ auto HandleStatement(Context& context) -> void {
       context.PushState(State::StatementIf);
       break;
     }
+    case Lex::TokenKind::Let: {
+      context.PushState(State::Let);
+      break;
+    }
     case Lex::TokenKind::Return: {
       context.PushState(State::StatementReturn);
       break;