[clangd] Add a tweak for filling in enumerators of a switch statement.

Add a tweak that populates an empty switch statement of an enumeration type with all of the enumerators of that type.

Before:
```
enum Color { RED, GREEN, BLUE };
void f(Color color) {
  switch (color) {}
}
```

After:
```
enum Color { RED, GREEN, BLUE };
void f(Color color) {
  switch (color) {
  case RED:
  case GREEN:
  case BLUE:
    break;
  }
}
```

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D88383