[FIR] Return K2 reference shortener result

This commit sets a list of `KtElement` as the return type of
`ShortenCommand::invokeShortening()`. It allows us to take the result
i.e., shortened PSIs. This can be used, for example, when we want to
run code-format only on the shortened PSI after running the reference
shortener.

^KT-57636 Fixed

analysis/analysis-api-fe10/src/org/jetbrains/kotlin/analysis/api/descriptors/components/KtFe10ReferenceShortener.kt

@@ -16,6 +16,7 @@ import org.jetbrains.kotlin.analysis.api.lifetime.KtLifetimeToken
 import org.jetbrains.kotlin.analysis.api.symbols.KtCallableSymbol
 import org.jetbrains.kotlin.analysis.api.symbols.KtClassLikeSymbol
 import org.jetbrains.kotlin.psi.KtDotQualifiedExpression
+import org.jetbrains.kotlin.psi.KtElement
 import org.jetbrains.kotlin.psi.KtFile
 import org.jetbrains.kotlin.psi.KtUserType
 
@@ -41,7 +42,7 @@ internal class KtFe10ReferenceShortener(
 
             override fun getQualifiersToShorten(): List<SmartPsiElementPointer<KtDotQualifiedExpression>> = emptyList()
 
-            override fun invokeShortening() {}
+            override fun invokeShortening(): List<KtElement> = emptyList()
         }
     }
 }