Use ReferenceVariantsHelper for code completion

[`ReferenceVariantsHelper`](https://github.com/JetBrains/kotlin/blob/master/idea/ide-common/src/org/jetbrains/kotlin/idea/codeInsight/ReferenceVariantsHelper.kt) bundles a lot of completion logic (e.g. for handling Java getters/setters) that would simplify the language server's completion generation.

Start with Kotlin's idea/ide-common/src/org/jetbrains/kotlin/idea/codeInsight/ReferenceVariantsHelper.kt and the language server's completion generation entry points. Compare the existing completion logic with the helper's handling of Java getters and setters. Done means the language server uses the helper where appropriate and completion behavior remains covered by its existing tests.

Written by the indexing model from the issue text.