Detect blocks that could be struct expr bodies

This approach lives exclusively in the parser, so struct expr bodies
that are syntactically correct on their own but are otherwise incorrect
will still emit confusing errors, like in the following case:

```rust
fn foo() -> Foo {
    bar: Vec::new()
}
```

```
error[E0425]: cannot find value `bar` in this scope
 --> src/file.rs:5:5
  |
5 |     bar: Vec::new()
  |     ^^^ expecting a type here because of type ascription

error[E0214]: parenthesized type parameters may only be used with a `Fn` trait
 --> src/file.rs:5:15
  |
5 |     bar: Vec::new()
  |               ^^^^^ only `Fn` traits may use parentheses

error[E0107]: wrong number of type arguments: expected 1, found 0
 --> src/file.rs:5:10
  |
5 |     bar: Vec::new()
  |          ^^^^^^^^^^ expected 1 type argument
  ```

If that field had a trailing comma, that would be a parse error and it
would trigger the new, more targetted, error:

```
error: struct literal body without path
 --> file.rs:4:17
  |
4 |   fn foo() -> Foo {
  |  _________________^
5 | |     bar: Vec::new(),
6 | | }
  | |_^
  |
help: you might have forgotten to add the struct literal inside the block
  |
4 | fn foo() -> Foo { Path {
5 |     bar: Vec::new(),
6 | } }
  |
```

Partially address last part of #34255.

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