JVM IR: fix isMarkedNullable for nullability-flexible types

Since nullability-flexible types in IR are represented by an annotation
on an IrSimpleType, not a special instance, the common implementation of
`KotlinTypeMarker.isMarkedNullable` in `TypeSystemContext` doesn't work.

This method is used for example to generate `typeOf` in JVM IR, in
`TypeSystemCommonBackendContext.generateTypeOf`, and this issue led to a
difference in behavior of `typeOf` for nullability-flexible types.

 #KT-44726 Fixed

compiler/testData/codegen/box/reflection/typeOf/flexibleType.kt

@@ -0,0 +1,27 @@
+// !USE_EXPERIMENTAL: kotlin.ExperimentalStdlibApi
+// TARGET_BACKEND: JVM
+// IGNORE_BACKEND_FIR: JVM_IR
+// WITH_REFLECT
+// FILE: box.kt
+
+package test
+
+import kotlin.reflect.*
+
+fun box(): String {
+    val v = returnTypeOf { J.get() }.toString()
+    if (v != "J") return "Fail: $v"
+
+    return "OK"
+}
+
+inline fun <reified T: Any> returnTypeOf(block: () -> T) =
+    typeOf<T>()
+
+// FILE: J.java
+
+public class J {
+    public static J get() {
+        return null;
+    }
+}