[clangd] Handle duplicate enum constants in PopulateSwitch tweak

If an enum has different names for the same constant, make sure only the first one declared gets added into the switch. Failing to do so results in a compiler error as 2 case labels can't represent the same value.

```
lang=c
enum Numbers{
One,
Un = One,
Two,
Deux = Two,
Three,
Trois = Three
};

// Old behaviour
switch (<Number>) {
  case One:
  case Un:
  case Two:
  case Duex:
  case Three:
  case Trois: break;
}

// New behaviour
switch (<Number>) {
  case One:
  case Two:
  case Three: break;
}
```

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D90555