Fix CCE in generated code that happens when comparing boxed chars

The problem was that when obtaining char from the wrapper,
codegen used int as expected type that led
to a ClassCastException: java.lang.Character cannot be cast to java.lang.Number

The solution is using coercion to chars, it's still correct,
because of implicit widening coercion in JVM from C to I

 #KT-15105 Fixed

compiler/testData/codegen/box/binaryOp/compareBoxedChars.kt

@@ -0,0 +1,16 @@
+fun <T> id(x: T) = x
+
+fun box(): String {
+    if (id('a') > id('b')) return "fail 1"
+    if (id('a') >= id('b')) return "fail 2"
+    if (id('b') < id('a')) return "fail 3"
+    if (id('b') <= id('a')) return "fail 4"
+
+    val x = id('a').compareTo('b')
+    if (x != -1) return "fail 5"
+
+    val y = id('b').compareTo('a')
+    if (y != 1) return "fail 6"
+
+    return "OK"
+}