K2: support implicit integer to unsigned conversions...

with dedicated opt-in language feature and special
annotation or module capability.
Not intended for a general use, solves specific K/N
scenario with interop libs.
#KT-55902 fixed

kotlin-native/backend.native/tests/build.gradle

@@ -5326,7 +5326,7 @@ standaloneTest("interop_zlib") {
 }
 
 standaloneTest("interop_objc_illegal_sharing") {
-    disabled = !isAppleTarget(project) || isK2(project) // KT-55902
+    disabled = !isAppleTarget(project)
     source = "interop/objc/illegal_sharing.kt"
     UtilsKt.dependsOnPlatformLibs(it)
     if (isExperimentalMM) {