No error if 'fun` keyword missing in abstract function in enums

```kotlin
public enum class TokenType(val type: String) {
ILLEGAL("ILLEGAL"),
/* ... */
RBRACE("{");

abstract parse(): Statement
}
```

This silently fails, no error on `abstract parse(): Statement` even though the `fun` keyword is missing. There is no error on the enum types either about a missing implementation. Only if the `fun`keyword is added, do they error correctly

Start by reproducing the Kotlin enum snippet from the issue and inspect how the language server reports syntax and abstract-member diagnostics. Trace the diagnostic entry point for the missing `fun` keyword and missing enum implementations. Done means both errors are reported consistently when the invalid snippet is analyzed.

Written by the indexing model from the issue text.