K2: introduce platform-specific visibility checks for overrides + tests

Related to KT-53197

compiler/testData/diagnostics/tests/visibility/notOverridingInternal.kt

@@ -0,0 +1,24 @@
+// FIR_IDENTICAL
+// MODULE: base
+// FILE: Base.kt
+package base
+
+abstract class Base {
+    fun foo() = internalFoo()
+
+    internal fun internalFoo() {}
+}
+
+// MODULE: impl(base)
+// FILE: Impl.kt
+package impl
+import base.*
+
+class Impl : Base() {
+    fun internalFoo() { /*not an override*/ }
+}
+
+fun foo() {
+    Impl().foo()
+    Impl().internalFoo()
+}