Revisit all @transient usages in coroutines (#4294)

Fixes #4293

kotlinx-coroutines-core/jvm/src/Exceptions.kt

@@ -24,9 +24,15 @@ public actual fun CancellationException(message: String?, cause: Throwable?) : C
 internal actual class JobCancellationException public actual constructor(
     message: String,
     cause: Throwable?,
-    @JvmField @Transient internal actual val job: Job
+    job: Job
 ) : CancellationException(message), CopyableThrowable<JobCancellationException> {
 
+    @Transient
+    private val _job: Job? = job
+
+    // The safest option for transient -- return something that meanigfully reject any attemp to interact with the job
+    internal actual val job get() = _job ?: NonCancellable
+
     init {
         if (cause != null) initCause(cause)
     }

kotlinx-coroutines-core/jvm/src/flow/internal/FlowExceptions.kt

@@ -2,6 +2,10 @@ package kotlinx.coroutines.flow.internal
 
 import kotlinx.coroutines.*
 
+/**
+ * Implementation note: `owner` is an internal marked that is used ONLY for identity checks by coroutines machinery,
+ * and it's never exposed, thus it's safe to have it both `@Transient` and non-nullable.
+ */
 internal actual class AbortFlowException actual constructor(
     @JvmField @Transient actual val owner: Any
 ) : CancellationException("Flow was aborted, no more elements needed") {

kotlinx-coroutines-core/jvm/src/internal/CoroutineExceptionHandlerImpl.kt

@@ -32,7 +32,11 @@ internal actual fun propagateExceptionFinalResort(exception: Throwable) {
 }
 
 // This implementation doesn't store a stacktrace, which is good because a stacktrace doesn't make sense for this.
-internal actual class DiagnosticCoroutineContextException actual constructor(@Transient private val context: CoroutineContext) : RuntimeException() {
+internal actual class DiagnosticCoroutineContextException actual constructor(context: CoroutineContext) : RuntimeException() {
+
+    @Transient
+    private val context: CoroutineContext? = context
+
     override fun getLocalizedMessage(): String {
         return context.toString()
     }