JVM_IR: use nullability when boxing/unboxing inline class types

Given inline class V(Any?), a coercion from (Object, V) to (Object, V?)
is boxing.

In theory, the same issue in the old backend can be fixed by making
`KotlinTypeMapper.mapUnderlyingTypeOfInlineClassType` use
`computeExpandedTypeForInlineClass`, but for some reason this breaks a
lot of stuff.

 #KT-48430 Fixed

compiler/testData/codegen/box/inlineClasses/boxNullableForFakeOverride.kt

@@ -0,0 +1,10 @@
+// IGNORE_BACKEND: JVM
+abstract class C<T> {
+    fun foo(v: T?, x: (T) -> Any?) = v?.let { x(it) }
+}
+
+inline class V(val value: Any?)
+
+class D : C<V>()
+
+fun box() = D().foo(V("OK")) { it.value } as String