[AA] Tests: Rewrite sealed class inheritors collection to use the declaration provider

- Now that binary libraries are decompiled to stubs instead of PSI
  files, we cannot collect sealed inheritors from `KtFile`s anymore.
  Since all `KtFile`s and binary library stubs are both indexed by the
  declaration provider, we can collect inheritors from its index
  instead.
- Invalidating all sessions at the end of `prepareSealedClassInheritors`
  fixes some improper resolve phases in lazy resolution test data. While
  the previous implementation requested an uncached resolve session, it
  didn't account for sessions of dependencies still being cached.

^KT-65960

analysis/analysis-api-providers/src/org/jetbrains/kotlin/analysis/providers/impl/KotlinStaticDeclarationProvider.kt

@@ -39,6 +39,7 @@ import org.jetbrains.kotlin.psi.stubs.KotlinClassOrObjectStub
 import org.jetbrains.kotlin.psi.stubs.elements.KtStubElementTypes
 import org.jetbrains.kotlin.psi.stubs.impl.*
 import org.jetbrains.kotlin.serialization.deserialization.builtins.BuiltInSerializerProtocol
+import org.jetbrains.kotlin.utils.addToStdlib.flattenTo
 import java.util.concurrent.ConcurrentHashMap
 
 public class KotlinStaticDeclarationProvider internal constructor(
@@ -396,6 +397,8 @@ public class KotlinStaticDeclarationProviderFactory(
     public fun getAdditionalCreatedKtFiles(): List<KtFile> {
         return createdFakeKtFiles
     }
+
+    public fun getAllKtClasses(): List<KtClassOrObject> = index.classMap.values.flattenTo(mutableListOf())
 }
 
 /**