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

compiler/testData/codegen/box/unsignedTypes/signedToUnsignedConversions.kt

@@ -0,0 +1,57 @@
+// WITH_STDLIB
+// !LANGUAGE: +ImplicitSignedToUnsignedIntegerConversion
+// IGNORE_BACKEND_K1: JVM
+// FILE: signedToUnsignedConversions_annotation.kt
+
+package kotlin.internal
+
+annotation class ImplicitIntegerCoercion
+
+// FILE: signedToUnsignedConversions_test.kt
+
+import kotlin.internal.ImplicitIntegerCoercion
+
+@ImplicitIntegerCoercion
+const val IMPLICIT_INT = 255
+
+@ImplicitIntegerCoercion
+const val EXPLICIT_INT: Int = 255
+
+@ImplicitIntegerCoercion
+const val LONG_CONST = 255L
+
+@ImplicitIntegerCoercion
+val NON_CONST = 255
+
+@ImplicitIntegerCoercion
+const val BIGGER_THAN_UBYTE = 256
+
+@ImplicitIntegerCoercion
+const val UINT_CONST = 42u
+
+fun takeUByte(@ImplicitIntegerCoercion u: UByte) {}
+fun takeUShort(@ImplicitIntegerCoercion u: UShort) {}
+fun takeUInt(@ImplicitIntegerCoercion u: UInt) {}
+fun takeULong(@ImplicitIntegerCoercion u: ULong) {}
+
+fun takeUBytes(@ImplicitIntegerCoercion vararg u: UByte) {}
+
+fun takeLong(@ImplicitIntegerCoercion l: Long) {}
+
+fun box(): String {
+    takeUByte(IMPLICIT_INT)
+    takeUByte(EXPLICIT_INT)
+
+    takeUShort(IMPLICIT_INT)
+    takeUShort(BIGGER_THAN_UBYTE)
+
+    takeUInt(IMPLICIT_INT)
+
+    takeULong(IMPLICIT_INT)
+
+    takeUBytes(IMPLICIT_INT, EXPLICIT_INT, 42u)
+
+//    such kind of conversions (Int <-> Long) actually are not supported
+//    takeLong(IMPLICIT_INT)
+    return "OK"
+}