Change name of Declaration node to be Identifier | ComputedPropertyName

This adds a type for ComputedPropertyName from ES6, and the necessary type modification in the compiler's Node hierarchy.
We should add a new type here:
```typescript
type DeclarationName = Identifier | LiteralExpression | ComputedPropertyName;
```
This type should be used throughout in all the places where you now write `Identifier | ComputedPropertyName`.