Introduce conversions from signed pure constants to unsigned ones

#KT-24717 In Progress
 #KT-25996 Open
 #KT-25997 Open

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

@@ -0,0 +1,23 @@
+// WITH_UNSIGNED
+// IGNORE_BACKEND: JS_IR, JVM_IR, JS
+
+fun takeUByte(u: UByte) = u.toByte()
+fun takeUShort(u: UShort) = u.toShort()
+fun takeUInt(u: UInt) = u.toInt()
+fun takeULong(u: ULong) = u.toLong()
+
+fun box(): String {
+    val b = takeUByte(200 + 55)
+    if (b != (-1).toByte()) return "Fail 1: $b"
+
+    val s = takeUShort(123)
+    if (s != 123.toShort()) return "Fail 2: $s"
+
+    val i = takeUInt(0xFFFF_FFFF)
+    if (i != -1) return "Fail 3: $i"
+
+    val l = takeULong(0xFFFF_FFFF_FFFF)
+    if (l != 0xFFFF_FFFF_FFFFL) return "Fail 4: $l"
+
+    return "OK"
+}