Migrate deprecations in Native compiler

Replacing deprecated Char.toInt() with Char.code and
Number.toChar() with Number.toInt().toChar(), where Number is not Int.

KT-23451

kotlin-native/Interop/Runtime/src/main/kotlin/kotlinx/cinterop/Utils.kt

@@ -479,12 +479,12 @@ private class U32CString(val chars: CharArray): CValues<IntVar>() {
         var indexIn = 0
         var indexOut = 0
         while (indexIn < chars.size) {
-            var value = chars[indexIn++].toInt()
+            var value = chars[indexIn++].code
             if (value >= 0xd800 && value < 0xdc00) {
                 // Surrogate pair.
                 if (indexIn >= chars.size - 1) throw IllegalArgumentException()
                 indexIn++
-                val next = chars[indexIn].toInt()
+                val next = chars[indexIn].code
                 if (next < 0xdc00 || next >= 0xe000) throw IllegalArgumentException()
                 value = 0x10000 + ((value and 0x3ff) shl 10) + (next and 0x3ff)
             }
@@ -527,7 +527,7 @@ public fun CPointer<ShortVar>.toKStringFromUtf16(): String {
     val chars = CharArray(length)
     var index = 0
     while (index < length) {
-        chars[index] = nativeBytes[index].toChar()
+        chars[index] = nativeBytes[index].toInt().toChar()
         ++index
     }
     return chars.concatToString()

kotlin-native/Interop/StubGenerator/src/main/kotlin/org/jetbrains/kotlin/native/interop/gen/CodeUtils.kt

@@ -71,7 +71,7 @@ fun String.quoteAsKotlinLiteral(): KotlinExpression = buildString {
         when (c) {
             in charactersAllowedInKotlinStringLiterals -> append(c)
             '$' -> append("\\$")
-            else -> append("\\u" + "%04X".format(c.toInt()))
+            else -> append("\\u" + "%04X".format(c.code))
         }
     }