[IrActualizer] Don't actualize expect types

This was never semantically correct, but was not important
before KT-63644, as they were immediately dropped after that anyway.

After KT-63644, they were used to compute fake overrides inside them,
which were later matched against actual class, which can
produce false-positive matching errors.

^KT-64835

compiler/testData/codegen/box/multiplatform/k2/covariantOverrideInActual.kt

@@ -0,0 +1,24 @@
+// LANGUAGE: +MultiPlatformProjects
+
+// MODULE: lib-common
+// FILE: common.kt
+
+
+abstract expect class Base
+expect class Child : Base
+
+
+// MODULE: lib()()(lib-common)
+// FILE: platform.kt
+
+actual abstract class Base() {
+    abstract fun foo(): Any
+}
+
+actual class Child: Base() {
+    override fun foo(): String = "OK"
+}
+
+fun box(): String {
+    return Child().foo()
+}