K2: fix NSME on toChar for subclasses of Number

Code in `ReplaceNumberToCharCallSitesLowering` relies on `Number.toChar`
being declared in IR as non-abstract. Otherwise the
`resolveFakeOverride` call there doesn't work, and the lowering can't
rewrite the call to `toInt().toChar()`, and we end up with a call to the
missing method `Number.toChar` in the bytecode.

 #KT-57217 Fixed

compiler/testData/codegen/box/primitiveTypes/numberToChar/javaSubclass.kt

@@ -2,9 +2,6 @@
 // ISSUE: KT-23447
 // WITH_STDLIB
 
-// IGNORE_BACKEND_K2: JVM_IR
-// Ignore reason: KT-57217
-
 // FILE: MyNumber.java
 
 public class MyNumber extends Number {