Do not report error about 'initializer required for @Transient properties' on lateinit vars

Fixes https://github.com/Kotlin/kotlinx.serialization/issues/553

plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/diagnostic/SerializationPluginDeclarationChecker.kt

@@ -114,7 +114,7 @@ open class SerializationPluginDeclarationChecker : DeclarationChecker {
                 filterUninitialized = false
             ) + declaration.primaryConstructorPropertiesDescriptorsMap(trace.bindingContext)
         propertiesMap.forEach { (descriptor, declaration) ->
-            val isInitialized = declarationHasInitializer(declaration)
+            val isInitialized = declarationHasInitializer(declaration) || descriptor.isLateInit
             val isMarkedTransient = descriptor.annotations.serialTransient
             val hasBackingField = descriptor.hasBackingField(trace.bindingContext)
             if (!hasBackingField && isMarkedTransient) {

plugins/kotlin-serialization/kotlin-serialization-compiler/testData/diagnostics/Transients.kt

@@ -6,4 +6,7 @@ import kotlinx.serialization.*
 @Serializable
 data class WithTransients(<!PLUGIN_ERROR("This property is marked as @Transient and therefore must have an initializing expression")!>@Transient val missing: Int<!>) {
     <!PLUGIN_WARNING("Property does not have backing field which makes it non-serializable and therefore @Transient is redundant")!>@Transient<!> val redundant: Int get() = 42
+
+    @Transient
+    lateinit var allowTransientLateinitWithoutInitializer: String
 }

plugins/kotlin-serialization/kotlin-serialization-compiler/testData/diagnostics/Transients.txt

@@ -3,6 +3,7 @@ package
 @kotlinx.serialization.Serializable public final data class WithTransients {
     public constructor WithTransients(/*0*/ missing: kotlin.Int)
     @kotlin.Deprecated(level = DeprecationLevel.HIDDEN, message = "This synthesized declaration should not be used directly", replaceWith = kotlin.ReplaceWith(expression = "", imports = {})) public /*synthesized*/ constructor WithTransients(/*0*/ seen1: kotlin.Int, /*1*/ serializationConstructorMarker: kotlinx.serialization.SerializationConstructorMarker?)
+    @kotlinx.serialization.Transient public final lateinit var allowTransientLateinitWithoutInitializer: kotlin.String
     @kotlinx.serialization.Transient public final val missing: kotlin.Int
     @kotlinx.serialization.Transient public final val redundant: kotlin.Int
     public final operator /*synthesized*/ fun component1(): kotlin.Int