Support of @DelegateAsync annotation

Grails 2.3 offers an AST transformation annotation `@DelegateAsync` that covers any service in async manner. More information here [http://grails.org/doc/latest/guide/async.html#promises](http://grails.org/doc/latest/guide/async.html#promises)
It would be great if IDEA support code completion and code navigation for such async services.

Example:
```
class SimpleService {
def activate() {

}
}
```
```
@DelegateAsync(SimpleService)
class AsyncSimpleService {
}
```
```
class HRequestService {
AsyncSimpleService asyncSimpleService
SimpleService simpleService

void activateHexagonRequest(HexagonRequest hexagonRequest) {
simpleService.activate()
asyncSimpleService.activate() // no code completion or code navigation
}
}
```

---

*Imported from [IDEA-133923](https://youtrack.jetbrains.com/issue/IDEA-133923) · Type: Feature · Votes: 0*
*Comments, attachments, dates and reporter are not part of the export — follow the link above for the full history.*

Start with the Grails @DelegateAsync documentation and the plugin's existing support for AST transformations, code completion, and navigation. Trace how delegated service methods are exposed to IntelliJ, then verify that AsyncSimpleService.activate() resolves for completion and navigation in the example.

Written by the indexing model from the issue text.