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