[pseudo] Eliminate a false parse of structured binding declaration.

Using the guard to implement part of the rule https://eel.is/c++draft/dcl.pre#6.

```
void foo() {
  // can be parsed as
  //   - structured-binding declaration (a false parse)
  //   - assignment expression
  array[index] = value;
}
```

Differential Revision: https://reviews.llvm.org/D132260