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/ir/irText/stubs/constFromBuiltins.__kotlin.Int.fir.ir.txt

@@ -22,7 +22,7 @@ CLASS IR_EXTERNAL_DECLARATION_STUB CLASS name:Int modality:FINAL visibility:publ
     annotations:
       IntrinsicConstEvaluation
     overridden:
-      public abstract fun toChar (): kotlin.Char declared in kotlin.Number
+      public open fun toChar (): kotlin.Char declared in kotlin.Number
     $this: VALUE_PARAMETER name:$this type:kotlin.Int
   FUN BUILTIN_CLASS_METHOD name:toByte visibility:public modality:OPEN <> ($this:kotlin.Int) returnType:kotlin.Byte
     annotations: